Objection 1. It would seem that synesis is not a virtue. Virtues are not in us by nature, according to Ethic. ii, 1. But synesis (judging well according to common law) is natural to some, as the Philosopher states (Ethic. vi, 11). Therefore synesis (judging well according to common law) is not a virtue.
Objection 2. Further, as stated in the same book (10), synesis (judging well according to common law) is nothing but "a faculty of judging." But judgment without command can be even in the wicked. Since then virtue is only in the good, it seems that synesis (judging well according to common law) is not a virtue.
Objection 3. Further, there is never a defective command, unless there be a defective judgment, at least in a particular matter of action; for it is in this that every wicked man errs. If therefore synesis (judging well according to common law) be reckoned a virtue directed to good judgment, it seems that there is no need for any other virtue directed to good command: and consequently prudence would be superfluous, which is not reasonable. Therefore synesis (judging well according to common law) is not a virtue.
On the contrary, Judgment is more perfect than counsel. But euboulia, or good counsel, is a virtue. Much more, therefore, is synesis (judging well according to common law) a virtue, as being good judgment.
I answer that, synesis (judging well according to common law) signifies a right judgment, not indeed about speculative matters, but about particular practical matters, about which also is prudence. Hence in Greek some, in respect of synesis (judging well according to common law) are said to be synetoi, i.e. "persons of sense," or eusynetoi, i.e. "men of good sense," just as on the other hand, those who lack this virtue are called asynetoi, i.e. "senseless."
Now, different acts which cannot be ascribed to the same cause, must correspond to different virtues. And it is evident that goodness of counsel and goodness of judgment are not reducible to the same cause, for many can take good counsel, without having good sense so as to judge well. Even so, in speculative matters some are good at research, through their reason being quick at arguing from one thing to another (which seems to be due to a disposition of their power of imagination, which has a facility in forming phantasms), and yet such persons sometimes lack good judgment (and this is due to a defect in the intellect arising chiefly from a defective disposition of the common sense which fails to judge aright). Hence there is need, besides euboulia (deliberating well), for another virtue, which judges well, and this is called synesis (judging well according to common law).
Reply to Objection 1. Right judgment consists in the cognitive power apprehending a thing just as it is in reality, and this is due to the right disposition of the apprehensive power. Thus if a mirror be well disposed the forms of bodies are reflected in it just as they are, whereas if it be ill disposed, the images therein appear distorted and misshapen. Now that the cognitive power be well disposed to receive things just as they are in reality, is radically due to nature, but, as to its consummation, is due to practice or to a gift of grace, and this in two ways. First directly, on the part of the cognitive power itself, for instance, because it is imbued, not with distorted, but with true and correct ideas: this belongs to synesis (judging well according to common law) which in this respect is a special virtue. Secondly indirectly, through the good disposition of the appetitive power, the result being that one judges well of the objects of appetite: and thus a good judgment of virtue results from the habits of moral virtue; but this judgment is about the ends, whereas synesis (judging well according to common law) is rather about the means.
Reply to Objection 2. In wicked men there may be right judgment of a universal principle, but their judgment is always corrupt in the particular matter of action, as stated above (Question 47, Article 13).
Reply to Objection 3. Sometimes after judging aright we delay to execute or execute negligently or inordinately. Hence after the virtue which judges aright there is a further need of a final and principal virtue, which commands aright, and this is prudence.
Source: Summa Theologica (New Advent)