It is as difficult to say what guesswork is as to say what the mind is, for guessing is not the action of any faculty of the mind; it is an action of the whole mind. Guesswork cannot be trained by exercises or studies; the quality of the guess, if not taken in utter ignorance (the educated guess), is always the quality of the mind as a whole. Guesswork has no abstract objects to practice on—indeed, in any formal test, the answers on which your mind is most used are those you guess at. Only in your guesses do you transcend the test, having in mind not only the subject matter but also the context of the test, weighing the character and reconstructing the thoughts of its makers, estimating your limits, knowing yourself and knowing the ways of the world.
A good guess is sibling to a good idea. Both trade risk for reward, certainty for power. To guess is often the only way to know something that others do not. A bad guess is always wrong; but that one guess is better than another also good is no reason to prefer it. Reality is recalcitrant and perverse. Reality has punished for laziness, and punished for effort; punished for absurdity, and punished for plausibility; punished for optimism, and punished for pessimism—and, of course, rewarded each at times. It leaves us no rules for guessing: we cannot guess with less than all our strength.