During these studious and observant years of teaching, I had slowly come to a settled conviction that I could say something of my own and something of consequence about Political Economy, especially at two points; and these two proved in the sequel to be more radical and transforming points than was even thought of at the first. For one thing, I had satisfied myself, that the word "Wealth," as at once a strangely indefinite and grossly misleading term, was worse than useless in the nomenclature of the Science, and would have to be utterly dislodged from it, before a scientific content and defensible form could by any possibility be given to what had long been called in all the modern languages the "Science of Wealth." Accordingly, so far as has appeared in the long interval of time since 1865, these "Elements" were the very first attempt to undertake an orderly construction of Economics from beginning to end without once using or having occasion to use the obnoxious word. A scientific substitute for it was of course required, which, with the help of Bastiat, himself however still clinging to the technical term "Richesse," was discerned and appropriated in the word "Value"; a good word indeed, that can be simply and perfectly defined in a scientific sense of its own; and, what is more important still, that precisely covers in that sense all the three sorts of things which are ever bought and sold, the three only Valuables in short, namely, material Commodities, personal Services, commercial Credits. It is of course involved in this simple-looking but far-reaching change from "Wealth" to "Value," that Economics become at once and throughout a science of Persons buying and selling, and no longer as before a science of Things howsoever manipulated for and in their market. For another thing, before beginning to write out the first word of that book, I believed myself to have made sure, by repeated and multiform inductions, of this deepest truth in the whole Science, which was a little after embodied (I hope I may even say _embalmed_) in a phrase taking its proper place in the book itself,--_A market for Products is products in Market_. The fundamental thus tersely expressed may be formulated more at length in this way: One cannot Sell without at the same instant and in the same act Buying, nor Buy anything without simultaneously Selling something else; because in Buying one pays for what he buys, which is Selling, and in Selling one must take pay for what is sold, which is Buying. As these universal actions among men are always voluntary, there must be also an universal motive leading up to them; this motive on the part of both parties to each and every Sale can be no other than the mutual satisfaction derivable to both; the inference, accordingly, is easy and invincible, that governmental restrictions on Sales, or prohibitions of them, must lessen the satisfactions and retard the progress of mankind.