The physical sciences are those which treat of the laws of matter, and of all complex phenomena in so far as dependent upon the laws of matter. The mental or moral sciences are those which treat of the laws of mind, and of all complex phenomena in so far as dependent upon the laws of mind. Most of the moral sciences presuppose physical science; but few of the physical sciences presuppose moral science. The reason is obvious. There are many phenomena (an earthquake, for example, or the motions of the planets) which depend upon the laws of matter exclusively; and have nothing whatever to do with the laws of mind. Many, therefore, of the physical sciences may be treated of without any reference to mind, and as if the mind existed as a recipient of knowledge only, not as a cause producing effects. But there are no phenomena which depend exclusively upon the laws of mind; even the phenomena of the mind itself being partially dependent upon the physiological laws of the body. All the mental sciences, therefore, not excepting the pure science of mind, must take account of a great variety of physical truths; and (as physical science is commonly and very properly studied first) may be said to presuppose them, taking up the complex phenomena where physical science leaves them. Now this, it will be found, is a precise statement of the relation in which Political Economy stands to the various sciences which are tributary to the arts of production. The laws of the production of the objects which constitute wealth, are the subject-matter both of Political Economy and of almost all the physical sciences. Such, however, of those laws as are purely laws of matter, belong to physical science, and to that exclusively. Such of them as are laws of the human mind, and no others, belong to Political Economy, which finally sums up the result of both combined. Political Economy, therefore, presupposes all the physical sciences; it takes for granted all such of the truths of those sciences as are concerned in the production of the objects demanded by the wants of mankind; or at least it takes for granted that the physical part of the process takes place somehow. It then inquires what are the phenomena of _mind_ which are concerned in the production and distribution [8] of those same objects; it borrows from the pure science of mind the laws of those phenomena, and inquires what effects follow from these mental laws, acting in concurrence with those physical one. [9] From the above considerations the following seems to come out as the correct and complete definition of Political Economy:--"The science which treats of the production and distribution of wealth, so far as they depend upon the laws of human nature." Or thus--science relating to the moral or psychological laws of the production and distribution of wealth."