DEAR BOY: The knowledge of mankind is a very use ful knowledge for everybody; a most necessary one for you, who are destined to an active, public life. You will have to do with all sorts of characters; you should, therefore, know them thoroughly, in order to manage them ably. This knowledge is not to be gotten systematically; you must acquire it yourself by your own observation and sagacity; I will give you such hints as I think may be useful land-marks in your intended progress. I have often told you (and it is most true) that, with regard to mankind, we must not draw general conclusions from certain particular principles, though, in the main, true ones. We must not suppose that, because a man is a rational animal, he will therefore always act rationally; or, because he has such or such a predominant passion, that he will act invariably and consequentially in the pursuit of it. No. We are complicated machines: and though we have one main-spring, that gives motion to the whole, we have an infinity of little wheels, which, in their turns, retard, precipitate, and sometimes stop that motion. Let us exemplify. I will suppose ambition to be (as it commonly is) the predominant passion of a minister of state; and I will suppose that minister to be an able one. Will he, therefore, invariably pursue the object of that predominant passion? May I be sure that he will do so and so, because he ought? Nothing less. Sickness or low spirits, may damp this predominant passion; humor and peevishness may triumph over it; inferior passions may, at times, surprise it and prevail. Is this ambitious statesman amorous? Indiscreet and unguarded confidences, made in tender moments, to his wife or his mistress, may defeat all his schemes. Is he avaricious? Some great lucrative object, suddenly presenting itself, may unravel all the work of his ambition. Is he passionate? Contradiction and provocation (sometimes, it may be, too, artfully intended) may extort rash and inconsiderate expressions, or actions destructive of his main object. Is he vain, and open to flattery? An artful, flattering favorite may mislead him; and even laziness may, at certain moments, make him neglect or omit the necessary steps to that height at which he wants to arrive. Seek first, then, for the predominant passion of the character which you mean to engage and influence, and address yourself to it; but without defying or despising the inferior passions; get them in your interest too, for now and then they will have their turns. In many cases, you may not have it in your power to contribute to the gratification of the prevailing passion; then take the next best to your aid. There are many avenues to every man; and when you cannot get at him through the great one, try the serpentine ones, and you will arrive at last.