In doing this, I confess, it has been impossible for me not to anticipate many things which, according to the plan I have laid down, will in some measure involve me in repetitions. I propose to investigate principles which are all relative and depending upon one another. It is impossible to treat of these with distinctness, without applying them to the objects on which they have an influence; and as the same principles extend their influence to several branches of my subject, those of my readers who keep them chiefly in their eye, will not find great variety in the different applications of them. In all compositions of this kind, two things are principally requisite. The first is, to represent such ideas as are abstract, clearly, simply, and uncompounded. This part resembles the forging out the links of a chain. The second is, to dispose those ideas in a proper order; that is, according to their most immediate relations. When such a composition is laid before a good understanding, memory finishes the work, by cementing the links together; and providing any one of them can be retained, the whole will follow of course. Now the relations between the different principles of which I treat, are indeed striking to such as are accustomed to abstract reasoning, but not near so much so, as when the application of them is made to different examples. The principle of self-interest will serve as a general key to this inquiry; and it may, in one sense, be considered as the ruling principle of my subject, and may therefore be traced throughout the whole. This is the main spring, and only motive which a statesman should make use of, to engage a free people to concur in the plans which he lays down for their government. I beg I may not here be understood to mean, that self-interest should conduct the statesman: by no means. Self-interest, when considered with regard to him, is public spirit; and it can only be called self-interest, when it is applied to those who are to be governed by it. From this principle men are engaged to act in a thousand different ways, and every action draws after it certain necessary consequences. The question therefore constantly under consideration comes to be, what will mankind find it their interest to do, under such and such circumstances? In order to exhaust the subject of political oeconomy, I have proposed to treat the principles of it in relation to circumstances; and as these are infinite, I have taken them by categories; that is, by the more general combinations, which modern policy has formed. These, for the sake of order, I have represented as all hanging in a chain of consequences, and depending on one another. See Book I. Chap. ii. I found this the best method for extending my plan, from which it is natural to infer, that it will also prove the best for enabling my readers to retain it.