[A] habit of choosing the mean appointed by reason as a prudent man would appoint it (ST I-II Q59 A1).In this we see the connection habit, virtue, reason, and prudence. A virtue is a kind of habit - specifically, a habit of choosing what a prudent man would reasonably choose.
The importance of reason in directing our choices and actions is critical. Reason is what distinguishes us from all other animals (and by "animal" I mean nothing more than what Aquinas and Aristotle before him meant: namely, a living being). It won't do for us to allow our actions to be guided by emotions or feelings or passions; we must use our brains to decide what we ought to do. For more on this, see this post. We act as humans (in the sense that St. Thomas means as described in that post) when we exercise free will to pursue a rationally determined end.
Okay then. We've examined habits and reason with respect to their role in our actions; we've defined virtue above; but what does St. Thomas say about prudence?
The Philosopher says (Ethic. vi, 5) that prudence is right reason applied to action (ST II-II Q47 A2).That's pretty short, but if mere repetition were any measure of a thing's importance to him then I'd have to say that this definition of prudence would be at or near the top of the list. Aquinas repeats this definition a lot. So let's get it into our heads. If we're going to be prudent, we've got to use our heads in thinking about what we do. By extension, the imprudent man does not apply right reason to action: instead, his actions may be directed by other things.
But this talk of applying reason to action could blur the most important feature of what virtue is: a habit of doing such things. But if a virtue is a habit, then it's not something that we necessarily have to think about in order to do. I don't have to think about whether to put my seatbelt on; it's something I just do (whether that qualifies as a virtue per se is not the point; the point is that it's habitual). It's also an action that a prudent man would perform.
A lot more needs to be said, obviously. This post is nothing more than purely introductory, because doing the right thing and exercising the virtues is necessarily more complex in many circumstances than the terribly simple question of wearing a seatbelt. But we can start from here. I'm not sure when I'll return to this particular topic, which is probably more than I'm capable of adequately addressing, but at least we've got what I think is a solid foundation here on which to build, whether in taking action ourselves in the Real World, or in future posts on moral theology.