Objection 1. It would seem that "euboulia, synesis, and gnome" are unfittingly assigned as virtues annexed to prudence. For "euboulia" is "a habit whereby we take good counsel" (Ethic. vi, 9). Now it "belongs to prudence to take good counsel," as stated (Ethic. vi, 9). Therefore "euboulia" is not a virtue annexed to prudence, but rather is prudence itself.
Objection 2. Further, it belongs to the higher to judge the lower. The highest virtue would therefore seem to be the one whose act is judgment. Now "synesis" enables us to judge well. Therefore "synesis" is not a virtue annexed to prudence, but rather is a principal virtue.
Objection 3. Further, just as there are various matters to pass judgment on, so are there different points on which one has to take counsel. But there is one virtue referring to all matters of counsel. Therefore, in order to judge well of what has to be done, there is no need, besides "synesis" of the virtue of "gnome."
Objection 4. Further, Cicero (De Invent. Rhet. iii) mentions three other parts of prudence; viz. "memory of the past, understanding of the present, and foresight of the future." Moreover, Macrobius (Super Somn. Scip. 1) mentions yet others: viz. "caution, docility," and the like. Therefore it seems that the above are not the only virtues annexed to prudence.
On the contrary, stands the authority of the Philosopher (Ethic. vi, 9,10,11), who assigns these three virtues as being annexed to prudence.
I answer that, Wherever several powers are subordinate to one another, that power is the highest which is ordained to the highest act. Now there are three acts of reason in respect of anything done by man: the first of these is counsel; the second, judgment; the third, command. The first two correspond to those acts of the speculative intellect, which are inquiry and judgment, for counsel is a kind of inquiry: but the third is proper to the practical intellect, in so far as this is ordained to operation; for reason does not have to command in things that man cannot do. Now it is evident that in things done by man, the chief act is that of command, to which all the rest are subordinate. Consequently, that virtue which perfects the command, viz. prudence, as obtaining the highest place, has other secondary virtues annexed to it, viz. "eustochia," which perfects counsel; and "synesis" and "gnome," which are parts of prudence in relation to judgment, and of whose distinction we shall speak further on (ad 3).
Reply to Objection 1. Prudence makes us be of good counsel, not as though its immediate act consisted in being of good counsel, but because it perfects the latter act by means of a subordinate virtue, viz. "euboulia."
Reply to Objection 2. Judgment about what is to be done is directed to something further: for it may happen in some matter of action that a man's judgment is sound, while his execution is wrong. The matter does not attain to its final complement until the reason has commanded aright in the point of what has to be done.
Reply to Objection 3. Judgment of anything should be based on that thing's proper principles. But inquiry does not reach to the proper principles: because, if we were in possession of these, we should need no more to inquire, the truth would be already discovered. Hence only one virtue is directed to being of good counsel, wheres there are two virtues for good judgment: because difference is based not on common but on proper principles. Consequently, even in speculative matters, there is one science of dialectics, which inquires about all matters; whereas demonstrative sciences, which pronounce judgment, differ according to their different objects. "Synesis" and "gnome" differ in respect of the different rules on which judgment is based: for "synesis" judges of actions according to the common law; while "gnome" bases its judgment on the natural law, in those cases where the common law fails to apply, as we shall explain further on (II-II, 51, 4).
Reply to Objection 4. Memory, understanding and foresight, as also caution and docility and the like, are not virtues distinct from prudence: but are, as it were, integral parts thereof, in so far as they are all requisite for perfect prudence. There are, moreover, subjective parts or species of prudence, e.g. domestic and political economy, and the like. But the three first names are, in a fashion, potential parts of prudence; because they are subordinate thereto, as secondary virtues to a principal virtue: and we shall speak of them later (II-II, 48, seqq.).
Source: Summa Theologica (New Advent)