But although we evolve truthful new conceptions from previous ones by these purely mental methods, there is a limit to the number capable of being evolved from a limited stock of ideas, because the number of combinations and permutations of such a stock, though usually large, are themselves limited. The number however is large in proportion to the degree of essential importance of the ideas, and is greatest when we employ those of the fundamental principles of nature, already referred to; for instance a greater number of new ideas have been evolved by means of appropriate mental processes from the law of gravitation and from that of electro-magnetism than from any minor truth in science. Persons therefore who are the least familiar with great demonstrable principles are usually the least able to conceive new truthful ideas of intrinsic importance, or to draw new verifiable inferences of much theoretical value. Every inventor and student knows that he continually requires new materials of thought, and if he does not obtain them, an obstacle, like a wall of adamant, rises before his mind in all directions, and prevents his forming new ideas. That also which is true of each individual is true of the collection of individuals, mankind; if new truths are not obtained, the thoughts of men flow in circles, and mental progress ceases. The mental characteristics of sequestered communities in remote isolated districts, are examples of this {166} fact. The influence of printing, railways, telegraphs, postal communication and other scientific developments, in aiding mental progress, afford other illustrations. A multitude of facts of this kind, and many others, leads us to the conclusion that each new idea requires a cause to produce it, and that human knowledge is subject to the great law of causation; also that the creation of an idea out of nothing would be a miracle, a phenomenon without a cause. Our present knowledge was not created by us, but was originated by previous knowledge and experience, including of course inherited impressions. Even in what is termed the "noblest effort of the mind," an act of reasoning or inference, we do not create an idea, but only render explicit in a new form of words, ideas already implicitly contained in the words of the propositions employed, as may easily be rendered manifest by mechanical means in Jevons's "Logical Machine;" a proper inference never contains more than its data. In the so-called "creation" of ideas by the imagination also, the new ideas are evolved from old ones, and rendered explicit by mental processes of analysis, combination, permutation, &c. Our scientific inventions also, being mental conceptions, an unlimited number of them cannot be made by means of a limited stock of old knowledge. It was in consequence of this limit, viz., the impossibility of actually creating ideas out of nothing, that human knowledge was not more advanced by metaphysical speculations