But though the objections against founding the science on hypothesis seem to me decisive, I do not give up hypothetical illustrations. Such illustrations not only make abstract reasonings more easily intelligible, they often expose their errors. Conclusions which appeared to be correct, when the vague terms of capital and labour, profit and wages, were used, are often found to be erroneous, when an hypothetical example embodies these abstractions, and endeavours to show the moral and physical processes by which the supposed result would be obtained. The absence of such illustrations is one of the great defects of Adam Smith. Perhaps this very defect contributed to the popularity of his work. Such illustrations, however useful, always give an appearance of stiffness and pedantry. The careless reader or hearer neglects them, and the real student is annoyed at having to learn the _dramatis personæ_ of an imaginary case. But if Smith had used them, he would probably have avoided some errors, and have preserved his successors from many more. His example in this and in some other respects, introduced a loose, popular mode of treating Political Economy, which has mainly retarded its progress. It may be remarked, that I have as yet used the word wealth, without defining it. I have done so, because I employ it in its popular sense, and because the ideas usually attached to that word appear to me to be sufficiently precise, to prevent any danger of my hearers misunderstanding it. Having now, however, completed the introduction to the science of Political Economy, having marked out its province, and stated the mode in which I intend to treat it, I think it advisable formally to define the term which expresses its subject matter. And this for two reasons. First, because, in a scientific work, every technical term ought to be defined; and, secondly, because that term has been employed by many of those who have preceded me, in senses differing from that which I adopt.