Since, then, Political Economy deals primarily with Persons, and only quite secondarily with Things, it is, under the definition and on every ground, a "moral science"; yet it must not be confounded in the least with what is sometimes called the science of Morals, or Ethics. There is one word that marks and circumscribes the field of Ethics, and that word is _Ought_; there is one word also that marks and circumscribes the field of Economics, and that word is _Value_. Now, the idea of _obligation_, on which ethical science is founded, and the idea of _gainful exchange_, on which economical science is founded, are totally distinct ideas. The imperatives of ethical obligation rest upon the consciences of men, and Duty is to be done at all hazards; guilt is incurred if it be neglected; while pecuniary gains and losses, however large, do not, or at least ought not, weigh a feather against an intuition of Right and Wrong. Economics, on the other hand, does not aspire to place its feet upon this lofty ethical ground; no man is ever under any moral obligation to make a trade; he properly makes it or not, according to his present sense of its gainfulness to himself; and so economic science finds a solid and adequate footing upon the expedient and the useful. Ethics appeals only to an enlightened conscience, and certain conduct is approved because it is Right, and for no other reason; Economics appeals only to an enlightened self-interest, and exchanges are made because they are mutually Advantageous, and for no other reason; each of the two Sciences, therefore, has a basis and sphere of its own, and the grounds of the two are not only independent, but also incommensurable. We will now apply _seriatim_ to Political Economy the four fundamental conditions belonging to all recognized Sciences, and so determine for ourselves whether it be not a strict science, and thus worthy in its leading propositions of all acceptation.