According to Aquinas and, I would dare say, any virtue theory that seriously considers human nature and what it needs to excel, there is a virtue of good guessing, called eustochia. More precisely, eustochia is the developed disposition to swift and likely conjecture, the aptitude for rapid discovery of congruities and incongruities. Not all conjectures, of course, are equal, and there is one form of conjecture that makes eustochia an especially interesting virtue. Reasoning, as we know, proceeds from a starting point to a terminal point, but it can't just be a series of stages. "The switch was flipped; the light must have gone on" is not an inference or bit of reasoning; it's just a series of claims. Reasoning or inference generally requires that we move from one claim to another by something conjoining them, whether explicit or not; this is called the middle term, and it's simply the means of getting from premise to conclusion. Thus, in "The switch was flipped, so the light must have gone on" there is an implicit middle term (or series of middle terms, it makes little difference) that links flipping the switch with the light going on. This middle term is the means of drawing a conclusion from the original premises or data. Eustochia is the virtue of being good at guessing; its most important form is being good at guessing what the best means of drawing a good conclusion is (Aquinas calls this form solertia, which is often translated as 'shrewdness', sometimes 'acumen').
There's no question that eustochia is a virtue, i.e., a disposition to excellence; but there are dispositions to excellence that one might consider only barely virtues, in that their excellence, although genuine, is in some sense secondary. One might consider modesty of dress an example; it's genuinely good, but it's a very context-sensitive goodness, and one might question how important it is in the grand scheme of living a life of moral and intellectual excellence. What puts eustochia very firmly in the virtue camp is that it is a form of excellence required by prudence, and prudence, as the good application of reason to action, is a foundational virtue. Prudence can't exist if we aren't, so to speak, ready for the unexpected, and the virtue of eustochia is precisely one of the ways we are. In that sense, it's worth our time to dwell a bit on eustochia and consider how it can be cultivated.
When virtues are linked to other, more important, virtues as being at least to some degree required by them, the associated virtues are called 'integral parts' of the more important virtues. Thus eustochia is an integral part of prudence. Because integral parts are drawn into a unity by the virtues to which they are annexed, as a general rule the integral parts of a virtue faciliate each other's operation and development. A good place to start our attempt to understand eustochia, then, is to look at how other integral parts of prudence contribute to it, and how it contributes to them; and also how they can compensate for deficiencies with regard to each other. One might divide the integral parts of a virtue in slightly different ways, but a good, standard version of such a division is that of Aquinas. On this view there are seven other integral parts of prudence: memory, understanding, docility (teachableness), reasoning, foresight, circumspection, and caution. I will only actually look at the relation to the first of these, memory.
Acting with prudence in a given case requires being able to draw on a good range of relevant memories. In this sense, we can think of a virtue of memory, i.e., the developed aptitude for retaining experiences in order to learn from them. This is a virtue we tend to play down because we are so reliant on external memory of various sorts; but in this we are fairly unusual. From ancient times across many different cultures this sort of disposition has been recognized as a virtue, and there is no question that it is integral to prudence: to apply reason to action well requires drawing on experience, which requires having relevant experiences to draw on, and memory is the virtue involved in having those relevant experiences. It's clear that memory plays an important role in facilitating conjecture; conjectures, however tenuous they may be, are never made out of nothing. If I am trying to figure out how to solve a new math problem, it's obvious that I am helped in doing so if I have solid experience with a wide range of other math problems. While I can't guarantee that any of them will be wholly relevant, there is a reasonable chance that these other types of math problem will have something in common with this new one, and that therefore what I've already experienced can provide at least an oblique guide -- a rough and read preliminary guide -- to what is completely new.
It is also plausible to suggest that eustochia facilitates the act of memory. One could perhaps imagine a creature that had an excellent memory storage, being potentially able to remember anything, but poor memory retrieval, rarely actually remembering things, because new experiences would not tend to recall relevant old experiences unless they were very, very alike. Such a creature would be confined to a limited range of action. Another limited creature might have similar excellent memory storage, but problematic memory retrieval for exactly the opposite reason, namely, because every new experience recalled a host of very of memories, many of which were not even remotely relevant. Certainly human beings who tend to diverge too much in either of these directions begin to have trouble functioning. A gift for good guessing has the ability to compensate for some divergence, i.e., slight weaknesses due either to slightly defective or excessive memory retrieval, by introducing a new train of association that might recall memories that are more relevant or by helping to weed out irrelevant ones.
Similarly, we can see again from the virtue of memory the importance of a virtue of happy conjecture. We do not have unlimited memories, and if we did, we could not handle them very well. Being able to guess well lets us get by with a more limited bank of memories than we would otherwise need, by allowing us as it were to compensate for the limitations of our memory and retained experience when the occasion requires it.
One mistake I think we must not make is assuming that guessing the means of inference is a rare thing. In fact, it's fairly routine. What's remarkable about those who have the virtue of eustochia, in the form of shrewdness about middle terms, is not the fact that they guess, which is unavoidable, but the fact that they consistently guess it well. (The requirement, of course, is not perfection but excellence.) The more fully the virtue of eustochia is developed the more guesses relevant to practical life move out of the realm of 'mere guesses' and into the realm of 'educated guesses'. An example might help clarify how eustochia differs from mere guessing.
In a complicated game, like chess, there are desired ends, e.g., to checkmate the king. These ends have certain features (e.g. the king cannot move without being in check by pieces on the board). These features help us to sort out the acceptable means. For instance, a series of plays that make impossible the features required for checkmate is ruled out. The actual path to checkmate must vary depending on your opponent's moves, but some moves are more apt than others to get one closer to the desired result, given the current configuration of the board and the possible and likely moves of one's opponent. Now, the possible moves between the first few moves and checkmate are astronomical (and depend, again, on events to some degree outside one's control, like the moves of one's opponent). Checking through each of them, or even whole sections of them, are not possible. What you need is the ability to guess your best move. This guessing can't be random or arbitrary, however, because it needs to approximate the best move with a good degree of consistency. A beginner lacks this consistency; he is almost random and haphazard. But a more expert chess player may have developed the knack to see what his best bets are even at a glance. He sees immediately the congruities and incongruities of the board, and can then exploit them to achieve the end he has in sight. This comes through extensive familiarity with the game, and an acquaintance with the strongest types of moves in a wide range of types of situation. The analogy is only a crude one. There are other aspects of chess that need to be factored in, and many, many other aspects of life. But shrewd guessing, the unreasoning, rapid selection involved in good guessing about how to get from A to B, is developed for practical life in general in something like the way it is developed in games: practice, and discipline, and building on native talents.
