Objection 1. It would seem that "synderesis" is a special power, distinct from the others. For those things which fall under one division, seem to be of the same genus. But in the gloss of Jerome on Ezekiel 1:6, "synderesis" is divided against the irascible, the concupiscible, and the rational, which are powers. Therefore "synderesis" is a power.
Objection 2. Further, opposite things are of the same genus. But "synderesis" and sensuality seem to be opposed to one another because "synderesis" always incites to good; while sensuality always incites to evil: whence it is signified by the serpent, as is clear from Augustine (De Trin. xii, 12,13). It seems, therefore, that "synderesis" is a power just as sensuality is.
Objection 3. Further, Augustine says (De Lib. Arb. ii, 10) that in the natural power of judgment there are certain "rules and seeds of virtue, both true and unchangeable." And this is what we call synderesis. Since, therefore, the unchangeable rules which guide our judgment belong to the reason as to its higher part, as Augustine says (De Trin. xii, 2), it seems that "synderesis" is the same as reason: and thus it is a power.
On the contrary, According to the Philosopher (Metaph. viii, 2), "rational powers regard opposite things." But "synderesis" does not regard opposites, but inclines to good only. Therefore "synderesis" is not a power. For if it were a power it would be a rational power, since it is not found in brute animals.
I answer that, "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. In order 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, proceeds from the understanding of certain things--namely, those which are naturally known without any investigation on the part of reason, as from an immovable principle--and ends also at the understanding, inasmuch as by means of those principles naturally known, we judge of those 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, bestowed on 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). Wherefore the first practical principles, bestowed on us by nature, do not belong to a special power, but to a special natural habit, which we call "synderesis". Whence "synderesis" is said to incite to good, and to murmur at evil, inasmuch 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.
Reply to Objection 1. The division given by Jerome is taken from the variety of acts, and not from the variety of powers; and various acts can belong to one power.
Reply to Objection 2. In like manner, the opposition of sensuality to "syneresis" is an opposition of acts, and not of the different species of one genus.
Reply to Objection 3. 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".
Source: Summa Theologica (New Advent)