It seems that synderesis is a sort of specific power distinct from others, for what falls under one division seems to be of one kind. But in Jerome's Gloss on Ezek. I, synderesis is divided from the irascible, the concupiscible, and the rational, which are sorts of powers. Therefore synderesis is a sort of power.
Further, opposites are of one genus. But synderesis and sensuality seem to be opposed, because synderesis always inclines to the good, but sensuality always to the bad, hence it is signified by the serpent, as is obvious from Augustine, De Trin. XII. It seems therefore that synderesis is a power, as is sensuality.
Further, Augustine says, in the book on free choice, that in natural judgment there are sorts of rules and seeds of virtues that are true and immutable, and these we call synderesis. Since, therefore, the immutable rules by which we judge pertain to reason according to its higher part, as Augustine says in De Trin. XII, it seems that synderesis is the same as reason. And so it is a sort of power.
But contrariwise, rational powers have themselves [se habet] to opposites, according to the Philosopher. But synderesis does not have itself to opposites, but is inclined only to good. Therefore synderesis is not a power. For it were a power, it would need to be a rational power, for its not found in beasts.
I respond that it must be said that synderesis is not power but habitude, though some have proposed that synderesis is a sort of power higher than reason, while others have said that it is reason itself, not as reason, but as nature. But to make this evident, it must be considered that, as was said above, human reason, as it is a sort of change, proceeds from intellection of some things, to wit, those naturally familiar [naturaliter notorum] apart from investigation of reason, as from a sort of immutable source. and it is also ended in intellection, inasmuch as we judge from sources naturally familiar to us through themselves [per se naturaliter nota], about those things which we discover by reasoning. Now it is sure that, just as reflective reason reasons about reflective matters, practical reasons reasons about workable matters. Therefore they must be instilled in us naturally, just as sources of reflective matters, sources of workable matters. Now the first sources of reflective matters naturally instilled in us do not pertain to any specific power, but to a sort of specific habitude, which is called intellection of sources, as is obvious in Ethic. VI. So also sources of workable matters naturally instilled in us do not pertain to a specific power but to a specific natural habitude, which we call synderesis. So too synderesis is said to incite to good and to grumble about bad, inasmuch as we proceed through first sources to discover, and to judge the discovered. It is obvious, therefore, that synderesis is not power but natural habitude.
To the first, therefore, it must be said that Jerome's division is directed toward diversity of acts, not diversity of powers. But different acts can be of one power.
To the second it must be said that likewise the opposition of synderesis and sensuality is directed to opposition of acts, not according to different species of one genus.
To the third it must be said that these immutable reasons are the first sources of workable matters, about which it does not happen to err; and they are attributed to reason as power and to synderesis as habitude. So too we naturally judge by both, to wit, reason and synderesis.
[Thomas Aquinas, ST 1.79.12, my translation. The Latin is here, the Dominican Fathers translation is here. ]
I should have done this article when discussing natural habitudes, but forgot; this will be an OK place to insert it, though.
'Synderesis', as one would expect from St. Thomas's comments, comes from St. Jerome; Jerome would have actually written syneidesis, which literally means co-knowing, and which he glosses as scintilla conscientiae, the spark of consciousness/conscience. Conscientia is the direct Latin translation of syneidesis, and they both literally mean 'co-awareness'. A corruption entered into the manuscript tradition, so syneidesis became synderesis.
St. Thomas distinguishes synderesis and conscience. He notes in the next article that synderesis is often called conscience by a figure of speech, but he reserves 'synderesis' for the habitude that is the understanding of practical principles and 'conscience' for the act that actually applies such principles in witnessing, judging, excusing, accusing, or punishing. Of course, in modern English, 'conscience' is used for both synderesis and conscience.
As a natural habitude, synderesis would fit into the taxonomy of natural habitudes as a natural habitude directed to operation according to the nature of the species, arising partly from nature, partly from external source, in apprehensive powers, just like its counterpart for speculative principles, and both are completed by intellectual virtues. For synderesis, this cultivation, especially of prudence, is what we are talking about when we talk about 'formation of conscience'.
