1. _Causes are abstract Conceptions._--WE have now to express, as well as we can, the fundamental character of that Idea of Cause of which we have just proved the existence. This may be done, at least for purposes of reasoning, in this as in former instances, by means of axioms. I shall state the principal axioms which belong to this subject, referring the reader to his own thoughts for the axiomatic evidence which belongs to them. But I must first observe, that in order to express general and abstract truths concerning cause and effect, these terms, _cause_ and _effect_, must be understood in a general and abstract manner. When one event gives rise to another, the first _event_ is, in common language, often called the cause, and the second the effect. Thus the meeting of two billiard-balls may be said to be the cause of one of them turning aside out of the path in which it was moving. For our present purposes, however, we must not apply the term cause to such occurrences as this meeting and turning, but to a certain conception, _force_, abstracted from all such special events, and considered as a quality or property by which one body affects the motion of the other. And in like manner in other cases, cause is to be conceived as some abstract quality, power, or efficacy, by which change is produced; a quality not identical with the events, but disclosed by means of them. Not only is this abstract mode of conceiving force and cause useful in expressing the fundamental principles of science; but it supplies us with the only mode by which such principles can be {185} stated in a general manner, and made to lead to substantial truth and real knowledge. Understanding _cause_, therefore, in this sense, we proceed to our Axioms. 2. First Axiom. _Nothing can take place without a Cause._