In Summa Theologica (I, Q79)Aquinas describes synderesis as a knowledge of first principles or an innate habit of thinking. It is a form of a priori reasoning. Synderesis cannot be mistaken. It produces discernment of the first practical principles, or primary precepts. We have an innate desire to "do good and avoid evil", which sums up in a general sense the more specific primary precepts: preservation of life, reproduction, living in society, education and worship of God.

Synderesis is not a power but a habit; though some held that it is a power higher than reason; while others [Cf. Alexander of Hales, Sum. Theol. II, 73] said that it is reason itself, not as reason, but as a nature. To make this clear we must observe that, as we have said above (Article 8), man's act of reasoning, since it is a kind of movement, comes from the understanding of certain things – those which are naturally known without any investigation on the part of reason, just like an immovable principle -and ends also at the understanding, because by means of those principles naturally known, we work out things which we have discovered by reasoning.

Now it is clear that, as the speculative reason argues about speculative things, so that practical reason argues about practical things. Therefore we must have, given us by nature, not only speculative principles, but also practical principles. Now the first speculative principles bestowed on us by nature do not belong to a special power, but to a special habit, which is called "the understanding of principles," as the Philosopher explains (Ethic. vi, 6). It follows that the first practical principles, given us by nature, do not belong to a special power, but to a special natural habit, which we call "synderesis". So "synderesis" is said to induce us to do good, and avoid evil, as through first principles we proceed to discover, and judge of what we have discovered. It is therefore clear that "synderesis" is not a power, but a natural habit.

Those unchangeable notions are the first practical principles, concerning which no one errs; and they are attributed to reason as to a power, and to "synderesis" as to a habit. Wherefore we judge naturally both by our reason and by "synderesis."

(ST I Q79 a12)

Extract from Aquinas, De Veritate 16

Hence it is that human nature, in so far as it comes in contact with the angelic nature, must both in speculative and practical matters know truth without investigation. And this knowledge must be the principle of all the knowledge which follows, whether speculative or practical, since principles must be more stable and certain. Therefore, this knowledge must be in man naturally, since it is a kind of seed plot containing in germ all the knowledge which follows, and since there pre-exist in all natures certain natural seeds of the activities and effects which follow. Furthermore, this knowledge must be habitual so that it will be ready for use when needed.
Thus, just as there is a natural habit of the human soul through which it knows principles of the speculative sciences, which we call understanding of principles, so, too, there is in the soul a natural habit of first principles of action, which are the universal principles of the natural law. This habit pertains to synderesis. This habit exists in no other power than reason, unless, perhaps, we make understanding a power distinct from reason.

Synderesis does not denote higher or lower reason, but something that refers commonly to both. For in the very habit of the universal principles of law there are contained certain things which pertain to the eternal norms of conduct, such as, that God must be obeyed; and there are some that pertain to lower norms, such as, that we must live according to reason. However, synderesis is said to refer to these unchangeable things in one way, and higher reason in another. For something is called unchangeable because of an immutability of its nature, and it is thus that divine things are unchangeable. Higher reason is said to deal with unchangeable things in this way. A thing is also said to be unchangeable because of the necessity of a truth, although the truth may concern things which according to their nature can change. Thus the truth: every whole is greater than its part, is unchangeably true even in changeable things. Synderesis is said to refer to unchangeable things in this way.

Further http://e-aquinas.net/pdf/stanciene.pdf

Professor Stanciene argues that Aquinas follows Albert the Great in seeing synderesis as part of a syllogism of the practical, moral intellect. This syllogism is a logical way of arguing which has a major premise followed by a statement and then a conclusion which follows. So we by nature incline to do good and avoid evil and habitually pursue good ends. The ends we pursue by nature are the primary precepts. It follows (conclusion) that the primary precepts are good.

The Dominican school elaborated its own theory of synderesis, based on the Christianized philosophy of Aristotle. Albert the Great (1193-1280) and Thomas Aquinas (1225-1274), the most prominent representatives of that school, proposed an intellective explanation of synderesis. Albert the Great had accepted Chancelier's definition of synderesis as potentia cum habitu but explained its infallibility by means of an Aristotelian idea, intellectus semper verus, applying it to the practical intellect or reason. This way Albert the Great unites the habit with the principles of natural law, and represents synderesis as the remnant of primordial human righteousness, proper to Adam before and concludes that on the practical level  these first principles correspond to the first principles on the speculative level. Therefore, synderesis is the anthropological substratum of these first principles of natural law.

Albert the Great explains that synderesis originates the major premise for practical reason but the minor premise is originated by the higher or lower reason. Practical reason uses this minor premise in concrete situations, but only together with dictamen of conscience. Since conscience is neither potency nor habit, but act, its mistakes in practical judgments have nothing in common with practical reason. This way, the spark of synderesis remains essentially infallible and inextinguishable.

The statements on synderesis by Thomas Aquinas, found in his Scriptum super libros Sententiarum, in De veritate, and in Summa Theologiae in essence repeat the concept of Albert the Great. He does not criticize the understanding of synderesis as potentia habitualis inherited from Chancelier. Nevertheless, at the beginning, until 1256 when he started his Quaestiones disputatae de veritate, he hesitated between this concept and the Aristotelian understanding of synderesis as mere habit. But in Summa Theologiae he defines synderesis as the innate habit of the first principles of practical reason. On these principles the judgments of the practical reason concerning particular actions are founded.

Since the nature of synderesis was widely discussed at the University of Paris, Thomas Aquinas considers it in the sixteenth chapter, De synderesi, of Quaestiones disputatae de veritate. There he answers three important questions: 1) Is synderesis a power or a habit? 2) Can synderesis err? 3) Are there some in whom synderesis is extinguished? To the first question he answers that synderesis is the power of reason which cannot be considered apart from every habit, for natural knowledge belongs to reason by reason of natural habit. The reply to the second and the third is negative: for probity to be possible in human actions, there must be some permanent principle which has unwavering integrity where there can be no error; although synderesis can be partially destroyed in a particular activity by violence, passion and sin, nevertheless its universal judgment can never be destroyed (a.3 in c).

Thus Aquinas relies on Aristotle and Albert the Great. He assumes that major premises of practical reason originate from synderesis, but minor premises spring from the higher or lower reason. He also makes a conclusion concerning the dictamen of conscience. The act of practical reason must be distinguished from the iudicium electionis of free will. They can be opposed to each other in the case of sinning. The relation of synderesis with other moral habits is based on cautiousness; for it gives knowledge to prudence: synderesis movet prudentiam, sicut intellectus principiorum scientiam.

Therefore, according to Thomas, a sinner has a speculative conscience of amorality of his behaviour, but in concretecircumstances that conscience is suppressed by passion, deception or otherfactors. The error in behaviour springs not from synderesis itself but from the way of its application to the situation. It springs from minor premises, generated by practical reason. It comes from the deliberate violation of moral demands; from inexcusable ignorance, which is the result of inexcusableneglect; from the excusable ignorance of some specific circumstances. In the latter case the error can be more or less pardonable.

Excusable ignorance alleviates guilt for bad behaviour; inexcusable, on the contrary, increases it. But inexcusable ignorance can be removed by learning the rules of morality. Trottmann maintains that in this way the problem of erring conscience, inherited from the twelfth century, was solved in the thirteenth century thanks to the philosophy of Aristotle. Errors in moral behaviour must be condemned, but those who err have to be forgiven, as far as forgiveness urges them to recognize and accept the truth, which corresponds to infallible synderesis, which cannot be extinguished even in the condemned.

Conclusion

In the thirteenth century two different concepts of synderesis, of will and intellect, were elaborated. The most prominent representatives of them are Bonaventura and Thomas Aquinas. The first maintained that synderesis is will with an innate striving for goodness, and conscience is a habit of practical reason. The second claimed that conscience is a member of the syllogism of practical reason, which gets its own principles from synderesis, the innate habit of practical reason. Aquinas underlines the affinity of synderesis with the angelic intellect by saying that synderesis corresponds more to the angelic direct intellectual cognition than to the human discursive rational cognition. Synderesis is the space where human nature meets angelic nature.

This meeting makes possible the participation of man in speculative and
practical angelic knowledge.

In this extract, James Fieser argues that Thomas Aquinas is following Aristotelean logic in his treatment of natural law.  So synderesis is the major premise of a logical syllogism.

Source: http://www.utm.edu/staff/jfieser/vita/research/aqu.htm

Natural Law

Thomas develops the idea of natural law most fully in his "Treatise on Law." He begins noting that law in general is a rational dictate aimed at the common good. There are four kinds of law: eternal law, natural law, human law and divine law. Eternal law, tracing back to Augustine,(6) is the unchanging divine governance over the universe. This covers both the general rules and particular cases. Natural law is a subset of eternal law which is obtainable by humans, but covers only general rules. Human law is an interpolation of natural law which is extended to particular cases. Finally, divine law (contained in the Bible) is a specially revealed portion of the eternal law which is meant to check possible errors in particular human laws.

Question 94 of the "Treatise on Law" directly addresses the concept of natural law in six articles. First, Thomas argues that our knowledge of the natural law is supplied by a habit of practical reason called "synderesis." Second, the natural law contains one key precept, from which subordinate precepts flow. Third, all virtuous acts in general are prescribed by the natural law, although their particular instances are not. Fourth, the most primary precepts of natural law are the same in all men, although the more particular secondary precepts are not (e.g. regarding theft). Fifth, the primary precepts of natural law cannot change, although the secondary derivations can change. Finally, humans cannot lose their knowledge of the primary precepts of natural law, although they can of the secondary ones.

In short, there is a hierarchical chain of principles involved in natural law, with the highest principle being its purest expression, and derived principles successively losing sanction. Throughout the "Treatise on Law," we find four tiers of these moral principles: the highest principle, primary principles, secondary principles of human law, and, finally, laws of nations.

The Synderesis Principle

As noted above, our knowledge of all natural law arises from synderesis, which is an inborn habit of our practical reason. Thomas explains this habit in three key places:

…the principles our nature imparts to us in practical matters do not belong to a special power but to a special habit there by nature, synderesis. And thus synderesis is said to incite us to good and to deter us from evil in that through first principles we both begin investigation and judge what we find. (1a Q. 79, 12)

Synderesis is called the law of our understanding inasmuch as it is the habit of keeping the precepts of natural law, which are the first principles of human activity. (Q. 94, 1)

Natural reason determines beforehand the ends of moral virtue by what is called synderesis…. (2a2ae, Q. 47, 6)
Thus, synderesis is a habit which keeps the precepts of natural law, and determines the ends of moral virtue. Thomas's account of the nature of synderesis is all too brief. He is content with simply noting that it is a weak instinctive habit, similar to religious faith (Q. 94, 1). This he contrasts with strong habits, such as acquiring language. He is, thus, concerned mainly with the phenomenon of synderesis, rather than with a psychological description. For modern readers of Thomas, though, this has advantages. For, by dealing only with the phenomenon of synderesis, we are not locked into an antiquated theology, metaphysics, or psychology, and may, thus, accept the synderesis intuition of natural law at face value. This is also important since nothing in his account of synderesis requires belief in God. God's existence becomes a factor only when we take into account the divine law which is meant to corroborate human laws which ultimately derive from synderesis.

The importance of synderesis is that all other moral principles are referred back (referuntur) to, and have a single root (una radice) in this most pure intuition (Q. 94, 2). Accordingly, the highest moral precept is often referred to by commentators as the synderesis principle. The content of the synderesis principle described in the following:

…every agent acts on account of an end, and to be an end carries the meaning of to be good. Consequently the first principle for the practical reason is based on the meaning of good, namely that it is what all things seek after. And so this is the first command of law, 'that good is to be sought and done, evil to be avoided'; all other commands of natural law are based on this…. As converging on one common primary precept these various precepts of natural law all take on the nature of one natural law. (Q. 94, 2)

The synderesis principle, then, is that "good is to be sought and done, and evil to be avoided." Two aspects of the above quote deserve attention. First, the meaning Thomas assigns to "good" is that which all things seek after. More specifically, what is sought after is a proper human end:

…the good or evil of an action, as of other things, depends on its fullness of being or its lack of that fullness. Now the first thing that belongs to the fullness of being seems to be that which gives a thing its species. (Q. 18, 2)(7)
Stated more clearly, the synderesis principle is that we should do that which is conducive to our proper end, and avoid those things which are not conducive to our proper end. Occasionally we may stray from our proper end as our will is ultimately attracted by what Thomas calls the apprehended or apparent good (as opposed to the true good; Q. 8, 1). However, human law, as drawn from the synderesis principle, aims specifically at aligning our apparent end with our true end. This procedure, as we shall see, involves not only the reflection of the agent, but of moral experts as well.(8)

The second important feature of the synderesis principle is that it contains two distinct parts:

1. If X is for a proper human end, then X ought to be done.
2. If X is not for a proper human end, then X ought not to be done.

At first glance, (1) and (2) appear the same. However, they are not logical equivalents, for the logical equivalent of (2) is,

• (2') If X ought to be done, then X is for proper human ends.

And, (2') is the converse, and not the equivalent of (1). Further, (1) is prescribing actions which we should do, and (2) is prescribing actions which we should avoid. Although the relation between virtues and natural law cannot be explored here, this distinction is important for Thomas in generating virtues to pursue, and vices to avoid. For convenience, (1) above will be referred to as the pursue virtue clause of the synderesis principle, and (2) the avoid vice clause.
Several commentators, such as Armstrong, Copleston, D'Arcy and O'Connor, have attacked the synderesis principle as being a vacuous tautology, "whose truth depends on the meaning of the terms used in expressing them."(9) D'Arcy, for example, believes this principle is analytically true. And, since tautologies say nothing about the facts involved in practical reasoning, the synderesis principle cannot serve as a major premise in practical reasoning. The function of the synderesis principle, according to D'Arcy, is that it is a purely formal principle "providing the rule that governs all moral reasoning, rather than its universal premise."(10)

Thomas does say that the synderesis principle "is based on the meaning of good" (Q. 94, 2). Initially this appears to support D'Arcy's interpretation. However, Thomas is only stressing his technical use of the word "good," as he intends it to mean for a proper end. This does not reduce the synderesis principle itself to a tautology. Consider the pursue virtue clause of the synderesis principle: "good ought to be done." This clause is tautological if and only if the definition of "good" includes "ought," or an obligation for action. But, nothing indicates that Thomas believes obligation is included in the concept of for a proper end.(11) Further, it is not unorthodox to give "good" and "ought" different meanings as, I suggest, is the case here. Hare and Mackie, for example, make strong distinctions between these two meanings.(12) Thus, the synderesis principle is not a tautology, as D'Arcy believes.

Armstrong argues similarly that "no person who used language in the normal way could say on the one hand 'X is good' but on the other 'you ought not do X'…. This principle, 'good ought to be done and evil ought to be avoided,' rather than being the first principle from which all others are derived [has] the status of being enshrined in all other natural law principle."(13) Although there is a slight error in Armstrong's wording of the problem, the gist of his attack is that we cannot say "X is good" yet "X need not be done."(14) But, given Thomas's definition of "good," the contradiction which Armstrong seeks does not arise. For, there is no definitional problem in saying, "X is for a proper human end" yet "you need not do X." Thomas, no doubt, believes this utterance is false. However, it is known to be false only through the synderesis principle. This is precisely why Thomas emphasizes the synderesis principle. Conceivably, a person without the synderesis principle could understand the concept of obligation, yet successfully pursue his end without knowing he was thereby fulfilling his obligation. This might be how Thomas would describe followers of rival moral systems, such as utilitarians. Thus, the job of the synderesis principle is to establish and continually reminds us of the link between human ends and obligation.

Granting that the synderesis principle is not analytic, how meaningful of a principle is it? Vernon J. Bourke argues that the synderesis principle has been misunderstood: "It does not tell man what are the good things that he should do; it is simply an obvious judgment that if you know that something is good for you to do, then you ought to do it."(15) Bourke's rejection of the synderesis principle rests on his interpretation that natural law is not the basis of Thomas's ethics.(16) Nevertheless, if the synderesis principle is only an obvious judgment, as Bourke suggests, it will have little application to moral judgments.

There are two ways that the synderesis principle is meaningful. First, like any first principle of morality, the synderesis principle is important in resolving moral dilemmas between secondary principles. As Mill suggests with his principle of utility, when a dispute arises between two principles (such as not lying and not harming others), we determine what is right according to the greatest happiness produced. With the synderesis principle, the right action would be the one which was most proper for human ends. In this sense, the synderesis principle is as meaningful as Mill's principle of utility.

The synderesis principle is also significant because of its appeal to the human telos, as opposed to utility or universalizability.(17) Accordingly, the more subordinate principles of morality become obligatory by being in accord with our proper end.

Subordinate Moral Precepts (secondary precepts) and Human Law

Thomas explains the connection between the synderesis principle and the primary precepts of morality in the following:

And so this is the first command of law, 'that good is to be sought and done, evil to be avoided'; all other commands of natural law are based on this. Accordingly, then, natural-law commands extend to all doing or avoiding of things recognized by the practical reason of itself as being human goods. (Q. 94, 2)
The most direct reading of the above is that anything we identify as being good (or for our proper end) is, thereby, something which ought to be done. This line of reasoning is clearly syllogistic, and may be more carefully formulated as follows:

1. All things which are for proper human ends are things we ought to do.
2. All X are things which are for proper human ends.
3. Therefore, All X are things we ought to do.

Germain G. Grisez argues that this interpretation misses Thomas's real position.(18) For Grisez, the precepts of natural law which stem from the synderesis principle are not imperatives, but only prescriptions. The difference is that imperatives would be rigid deductions which progressively enclose submissive subjects in "an ever-shrinking pen." Prescriptive precepts, on the other hand, allow the agent uninhibited spontaneity over all possibilities of life, guided by his telos. However, it is doubtful whether any moral prescript can allow uninhibited spontaneity as Grisez suggests. Violations of moral precepts are backed by punishments, and in this light, even prescriptions would enclose submissive subjects in an ever-shrinking pen. Consequently, Grisez is offering an interpretation which suffers from the same constraints as the interpretation he attacks.

In any case, several aspects of Thomas's discussion suggest that deductions from the synderesis principle are syllogistic. The first involves the relation between the synderesis principle and the subordinate primary principles. Thomas argues that we discover our telos by considering our most basic human natural inclinations (Q. 94, 2). These he lists according to the three Aristotelian faculties of the human soul. Corresponding to the vegetative faculty, we have an inclination for self-preservation. From our appetitive faculty, we have an inclination toward heterosexual activities, and an inclination to educate our offspring. Finally, according to our rational faculty, we have inclinations to be rational, know God, and live in society. For Thomas, these six inclinations comprise what is most proper for humans, and provide the basis for the primary precepts of morality.

From these natural inclinations, six primary precepts emerge:

1. Preserve human life
2. Have heterosexual (as opposed to homosexual) intercourse
3. Educate your children
4. Shun ignorance
5. Worship God
6. Avoid offending others

(20) Thomas notes specifically that the divine law corroborates the last two of these: "thou shalt love the Lord thy God," and "thou shalt love thy neighbor" (Q. 100, 3 and 11).(21) This procedure of arriving at primary principles through natural inclinations is syllogistic in nature. Natural inclinations are brute facts of human existence. To recognize our moral obligation from these inclinations, there must be logical connection between these, and the fact that obligation is linked to our telos. The following illustrates this logical connection with the primary principle of avoiding harmful acts:

1. All things which are unsuitable for human ends are things one ought not do. [vice clause of the synderesis principle]
2. All harmful acts are things which are unsuitable for human ends. [based on the observation that humans instinctively live in society]
3. Therefore, all harmful acts are things one ought not do. [primary principle]

Another aspect of Thomas's account which indicates a syllogistic structure is that involving secondary precepts. Technically, when formulating further precepts from the primary ones, we enter the domain of human law. There are two kinds of human law (Q. 95, Art. 2). One kind involves a loose deduction based on customary practices, such as the specific manner in which someone should be punished. This type of law has no authority above the human institution itself. A second type of human law is more fundamental. This involves a strict derivation from primary principles, and carries some force of natural law. This second and more strict deduction is "like to that by which, in the sciences, demonstrated conclusions are drawn from the principles." The thrust of these and similar passages is that he is speaking of deduction in a technical, and not a metaphorical manner (c.f. Q. 94, 4 and 6; 99, 2; and 100, 3 and 11). He notes as an example that "some things are therefore derived from the common principles of the natural law by way of conclusions: e.g. that one must not kill may be derived as a conclusion from the principle that one should do harm to no man" (Q. 95, 2). The deduction here is,

1. All acts which harm others are acts we ought not do. [from a primary principle of natural inclination]
2. All acts of unjustified killing are acts which harm others.
3. Therefore, All acts of unjustified killing are acts we ought not do.

When deducing secondary principles of human law, any point of departure makes a perversion in its connection with natural law. For this reason, unlike the primary principles which can be deduced by everyone,(22) secondary principles of human law require "careful reflection of wise men" (Q. 100, 3). To guard against corruption on this level, divine law has corroborated the most general precepts of human law in the 10 commandments (Q. 100, 4 and 11).

The syllogistic structure of practical reasoning can also be seen in Thomas's account of the final tier of principles of human law, which may be called tertiary principles. As with secondary principles of human law, there are two sorts of tertiary principles. First, there are civil laws which are customary, and obtain their authority from the governing institution itself (Q. 95, 4, cf. 100, 11; and 104, 1). Second, there are the laws of nations (jus gentium)(23) which involve strict derivation from secondary principles. To the laws of nations "belong those things which are derived from the law of nature as conclusions from principles, e.g., just buyings and sellings, and the like, without which men cannot live together; and this belongs to the law of nature, since man is by nature a social animal…" (Q. 95, 4). Like the secondary prohibitions against theft, the tertiary prohibitions against fraud are prescribed by statesmen who are experts in matters of law. Again, these tertiary principles carry some force of natural law. The sanction of natural law inevitably becomes diminished as we proceed to more particular principles, since these depend on cultural influences. Plagiarism, for example, might not be considered stealing in some cultures. Nevertheless, the general notion of fraud would still be a species of stealing, and thus violate natural law.

Beginning with the "avoid vice" clause of the synderesis principle, the deduction of primary, secondary, and tertiary principles is as follows:

1. All things which are unsuitable for human ends are things one ought not do. [synderesis principle]
2. All harmful acts are things which are unsuitable for human ends. [based on the inclination of humans to live in society]
3. Therefore, all harmful acts are things one ought not do. [primary principle]
4. All acts of stealing are harmful acts. [carefully reflected observation of wise men]
5. Therefore, all acts of stealingare things one ought not do. [secondary principle]
6. All acts of fraud are acts of stealing. [observation by legal experts]
7. Therefore, All acts of fraud are things one ought not do. [law of nations]

This interpretation casts natural law in a new light. Thomas frequently warns about departing from natural law when devising human law. The above account clearly indicates where departures may take place: by invalid deduction, or by supplying an inappropriate minor premise. Because the AAA-1 syllogistic form used in the above chain is fairly intuitive, departures through invalid deduction are not a major threat. Often it would suffice to note, as Thomas does, that "one must not kill may be derived as a conclusion from the principle that one should do harm to no man." The real problem is in supplying the minor premise in each of these, without which subordinate moral principles cannot be deduced. The task of supplying the proper minor premise is so difficult, Thomas argues, that experts in law are needed for the more precise ones and, further, divine law is supplied to check even their mistakes. Although a strict formulation of the chain of practical reasoning may not always be necessary, it is nevertheless the ideal form of deliberation against which our more informal moral decisions must be weighed. It is also the clearest way of seeing where minor premises are involved. In comparison with interpretations which view the logic of natural law as metaphorical, the above account takes some of the guess work out of practical reasoning.

One final syllogism remains to complete Thomas's syllogistic chain of practical reasoning: the practical syllogism. It is by means of this final syllogism that laws of nations lead immediately to an agent's action.

The Logical Nature of Practical Syllogisms

Thomas's practical syllogism (syllogismus operativus) has its origin in Aristotle. Although Thomas refers to the practical syllogism several times in his writings,(24) his fullest explanation is the following from Q. 76, 1:

Reason directs human action with two types of knowledge, general knowledge and knowledge of particulars. In deciding to act, the mind constructs a syllogism the conclusion of which is a judgment or a choice. Actions exist in individual situations. Hence, the conclusion of this practical syllogism is singular. We cannot conclude to a particular application of a general premise unless the situation fits the general definition, e.g. to avoid parricide a man would have to know not only that it was wrong but also that the intended victim [hunc] was his father. Ignorance of either of these truths could be the cause of an act of parricide; one would be ignorance of a general principle by which reason is directed, the other would be ignorance of a particular circumstance.
As with Aristotle, a practical syllogism is the final act of practical reasoning by which we initiate a moral action. It consists of a universal major premise (a moral principle), a particular minor premise (an observation about an agent's situation) and a particular conclusion (an agent's doing a particular action).(25)

O'Connor construes Thomas's account of the practical syllogism quite generally, as being an outline of the entire process of practical reasoning. For O'Connor, where X is any morally significant action, we reason that,

• MAJOR PREMISE – X is right (or wrong)
• MINOR PREMISE – This is a case of X
• CONCLUSION – This ought to be done (or avoided)

However, in the above quote, Thomas suggests that the practical syllogism's function in practical reasoning is more restricted: "…to avoid parricide a man would have to know not only that it was wrong but also that the intended victim was his father." Or,

• MAJOR PREMISE – It is wrong to kill one's father.
• MINOR PREMISE – This intended victim of killing is the agent's father.
• CONCLUSION – The agent is restrained from an act of parricide.

Contrary to O'Connor's interpretation, the major premise here is fairly specific.(26) In fact, the major premise is a clear reference to a law of nations, since the prohibition against parricide follows from the secondary principle that we should honor our parents. The practical syllogism, then, dovetails with the syllogism involving laws of nations and is thus the concluding syllogism in the syllogistic chain which begins with the synderesis principle.
The standard interpretation of both Aristotle and Thomas is that the practical syllogism only superficially resembles a logical syllogism. Thomas Gilby, for example, argues that Thomas's practical syllogism does not follow strict syllogistic rules:

…"syllogism" here does not mean the demonstrative or apodictic syllogism … but a syllogism according to Aristotle's general definition of an argument where, given a certain thing, something further is bound to follow.
Gilby argues that a practical syllogism differs from more demonstrative syllogisms (such as scientific ones) in that moral decisions are subjective. This subjectivity arises from the fact that (a) information pertaining to the minor premise is "played on by the appetite," and (b) more information is continually being gathered which "cannot be comprehended in the premises."(27) But Gilby has not made his point, even when granting the subjective nature of the minor premise in a practical syllogism. He seems to be making a basic confusion between the validity of an argument's form and our knowledge of the truth of an argument's premises. Whereas the subjective nature of the minor premise in a practical syllogism may distinguish it from premises in scientific syllogisms (which are more absolute), this has no bearing on whether formal syllogistic rules are used in practical syllogisms.

More serious arguments against the logical nature of practical syllogisms arise from discussions of Aristotle's practical syllogism, but apply equally to Thomas's account. There are two main objections, one arising from the fact that the minor premise is a perception, and not a proposition, and the other arising from the fact that the conclusion is an action, and not a proposition. The general criticism in both cases is that logical arguments can contain only propositions, and not things like perceptions or actions. I will argue that both of these objections are surmountable for Thomas.

John Cooper has most recently advanced the first objection against Aristotle's formulation of the practical syllogism. According to Cooper,

… the minor premise is provided by an act of perception…. Now it is this connection with perception that makes one doubt whether this so-called [practical] syllogism is a syllogism–that is, an argument–at all.(28)
Cooper argues that the act of perception in the minor premise should not be interpreted as providing a piece of information which leads the agent to choose the right action. Instead, Aristotle is describing a psychological link in a chain leading from a decision to act (major premise), to a perception (minor premise) and finally to an action (conclusion). There is, then, no argument involved, but only a psychological chain of events which, when schematized, resembles the structure of a syllogism. Cooper's conclusion is that the practical syllogism is not a reasoning process, and should not be considered part of moral deliberation.

The success of Cooper's interpretation hinges on whether the perception in the minor premise provides information to be reasoned about. D.K. Modrak argues that, for Aristotle, such information is provided.(29) Modrak bases her argument on the notion of a propositional attitude–the attitude that such and such is the case. Typical propositional attitudes include "believing that…," "desiring that…," "saying that…," "wishing that…" and "fearing that…." According to Modrak, the propositional attitude indicated in the minor premise of the practical syllogism is perceptual–perceiving that such and such is the case.(30) For this reason (and contrary to Cooper), the practical syllogism is a genuine argument since each line, including the minor premise, indicates a proposition. Although Modrak's interpretation has been criticized for misrepresenting Aristotle's intentions,(31) she has, nevertheless, provided an interesting alternative to Cooper's non-propositional interpretation of practical syllogisms.

Which of these two interpretations best represents Thomas's view of practical syllogisms? The illustration with parricide, again, is,

• MAJOR PREMISE – It is wrong to kill one's father.
• MINOR PREMISE – This intended victim of killing is the agent's father.
• CONCLUSIONS – The agent is restrained from an act of parricide.

The ostensive reference "this man" (hunc) in the minor premise suggests that, like Aristotle, the minor premise involves a perception. The question is whether it is only a perception, on Cooper's interpretation, or whether it is a perception of a proposition (a perception that such and such is the case) on Modrak's interpretation. Modrak's interpretation seems favorable for two reasons. First, Thomas's account of practical syllogisms in Q. 76, 1 notes that, "Reason directs human action with two types of knowledge, general knowledge and knowledge of particulars." The general knowledge refers to the major premise, and the particular knowledge refers to the minor premise. What is important is that the minor premise involves an item of knowledge which aids in directing action. Hence, the perception that "This intended victim is my father" in the minor premise provides the agent with some propositional knowledge.

Second, Thomas's account of the practical syllogism clearly involves deliberation, for the agent's final decision for restraint arises from his knowledge of the major and minor premise. By contrast, on Cooper's psychological interpretation of the practical syllogism, the final decision for restraint would culminate in the major premise itself. From there on, there is no deliberation–only the movement of psycho-physiological cogs.(32) Thus, a propositional interpretation of the minor premise is to be preferred over a non-propositional interpretation. So far, the practical syllogism should be construed as follows:

• MAJOR PREMISE – All acts of killing one's father are things one ought not do. [law of nations]
• MINOR PREMISE – Minor premise: This intended victim of killing is the agent's father. [perception]

  (2') Minor premise: Some act of killing this man is (or would be) an act of killing one's father. [proposition indicated by (2)]

• CONCLUSION – The agent is restrained from killing his father. [action]

The second, and most common objection raised against the logical status of practical syllogisms is that its conclusion is an action, as opposed to a proposition. Regarding the conclusions of practical syllogisms, Thomas notes in Q. 76, 1 that "Conferens enim de agendis, utitur quodam syllogismo, cujus conclusio est judicium seu electio vel operatio."(33) There is much flexibility here in the nature of a conclusion to a practical syllogisms. It may be a "judgment (or, in other words, a choice) or an action." Insofar as they are actions, though, the criticism would seem to be appropriate: how can the conclusion to an argument be an action?

One solution to this problem was recently given by Robert Audi. According to Audi, the conclusion of a practical syllogism is a judgment that the agent should do a particular act. This is based on Audi's conviction that the conclusion of a practical syllogism must be an appropriate response to the practical question "what should I do?"(34) Clearly, many actions would be appropriate responses to this question. For example, the act of contributing to UNICEF would be an appropriate response to the question, "What should I do about the problem of world hunger?" This approach would seem to cover all morally relevant actions, insofar as they answer the question, "What should I do about such and such?" However, Audi's account tells us only that moral actions involve judgments. He does not show us how moral actions indicate propositions.

A solution may be found in the writings of William Wollaston, the 18th century British rationalist. For Wollaston, those propositions are true, which express things as they are: or, truth is the conformity of those words or signs, by which things are expressed, to the things themselves…. It is certain there is a meaning in many acts and gestures. Everybody understands weeping, laughing, shrugs, frowns, etc., these are a sort of universal language.

From this, Wollaston concludes that propositions may be indicated by words, signs and actions.(35) Wollaston believes that this view of actions-as-propositions is particularly common with morally relevant actions. For example, if I drive away in a particular automobile, I am indicating the proposition that "I have the right to drive away in this vehicle." If I own the automobile, this action indicates a true proposition. If, on the other hand, I am stealing an automobile, then this action indicates a false proposition.

But how, more precisely, would an action indicate a proposition on Wollaston's account? Clearly, Wollaston's theory makes sense only when a particular action is deliberate or intentional. For example, if while sleepwalking I drive away in an automobile, my behavior would not indicate the proposition that "I have the right to drive away in this automobile." Thus, the focus of Wollaston's theory is on my belief states and intentions as they form the foundation of my action, and not, strictly speaking, the physical action itself. Consequently, my physical behavior serves as an indicator of my belief states, and these, in turn, indicate true or false propositions.

This may be what Thomas has in mind when claiming that the conclusion to a practical syllogism may be an action. With the parricide example, the agent's concluding act is one of restraint from killing his father. This indicates the agent's belief state that to kill his father would be wrong, and this, in turn, indicates the proposition that "killing this man is wrong." On this analysis, the practical syllogism about parricide would be as follows:

1. All acts of killing one's father are things one ought not do. [law of nations]
2. This intended victim of killing is the agent's father. [perception]

   (2') Some act of killing this man is (or would be) an act of killing one's father. [proposition indicated by (2)]

3. The agent is restrained from killing his father. [action which presupposes a belief state]

   (3') Some act of killing this man is a thing one ought not do. [proposition indicated by the belief state in (3)]

I have argued that moral reasoning for Thomas involves a chain of logical syllogisms beginning with the synderesis principle, and concluding with an act of a practical syllogism. Particularly, I have shown that the synderesis principle "we ought to do good and avoid evil" is not tautological, and is our ultimate source of knowledge of the natural law. Also, I have argued that subordinate moral principles are syllogistically derived from the synderesis principle, and, to varying degrees, carry the force of natural law. Finally, I have shown that the concluding practical syllogism of this argument chain can be reasonably interpreted as a logical syllogism.

In conclusion, it should be noted how this scheme of practical reasoning affects the individual moral agent. As individuals, we are directly accountable for our most general intuition of natural law through synderesis, and for our final actions determined by practical syllogisms. We are first accountable for understanding our obligation to act in accord with proper human ends, the most clear ends involving honoring God, and living in society. Lawmakers, along with scripture, provide us with particular laws governing of our moral obligation. Although we are bound by these laws, we are not responsible for understanding how these relate to our end. Our final direct responsibility concerns our judgment about whether our actions are in accord with these particular laws. We are responsible for perceiving that an act in question is in accord with tertiary principles and, ultimately, whether the act is in accord with our obligation.