(3) _Introspection._ Everybody buys and sells, and almost everybody watches the action of his own mind enough to see what are his _motives_ in buying and selling, and soon comes to know also that the other party has corresponding motives. Even the child knows perfectly, that it takes two to make a bargain, that each party renders something to the other, that each is glad to part with something for the sake of receiving something from the other, and that this higher esteem put by each on what is taken from the other makes for each the gain of the trade. A very little introspection tells anybody, that were this higher esteem wanting in the minds of either of the two, the trade would not take place at all. Everybody within the pale of _compos mentis_ knows, that, were his own desire for the rendering of another to increase, he himself would offer more of his own rendering rather than forego the trade; and he rightly infers, that what is true of himself is true of all other men; and so, every seller rightly tries to display his wares in such a way as to increase the desire of buyers for them; knowing full well from his own experience in buying that, other things being equal, they will be willing to render him more for them in consequence. The phrase above, "rightly infers," is based upon the truth, that all men are remarkably alike in certain great departments of action; and that, in no department are they so nearly alike as in this of buying and selling. Introspection, therefore, an easy self-knowledge open to all persons alike, and a personal experience in these matters that everybody gains, give most trustworthy answers to Inductive inquiry along these lines. Trade is natural and gainful, as any person can see, who stops to ask himself why he has made, or is about to make, a given trade; and if natural and gainful to _him_, equally so for precisely the same reasons to the party of the other part; hence no law or encouragement is needed to induce any persons to enter upon traffic; and any law, or artificial obstacle, that hinders any two persons from trading, who would otherwise trade, not only interferes with an inalienable right that belongs to both, but also destroys an inevitable gain that would otherwise accrue to both. Political economy is very fortunate, accordingly, in being able to make its appeal to the common sense of all men, giving sound starting-points through self-knowledge possessed by all men, guiding to safe steps by means of Induction all who like to generalize and prove, and especially breaking up current fallacies by asking the potent question, "How would you like it yourself?"