3. And, finally, Dr. Tyndall tells us that "The expectation of likeness [_i. e._, uniformity] in the procession of phenomena is not that on which the scientific mind founds its belief in the order of nature. If the force is _permanent_, the phenomena are _necessary_ whether they resemble or do not resemble any thing that has gone before. Hence in judging of the order of nature our inquiry eventually relates to the permanence of force,"[441] or, as he elsewhere styles it, "the conservation of energy," which means "that no power can make its appearance in nature without an equivalent expenditure of some other power; that natural agents are so related as to be mutually convertible, but that no new agency is created."[442] Whether this is or is not a correct statement of the principle of the conservation of energy we shall see by and by. And now, after having hunted the game through many tortuous passages to its final burrow, what have we found? That the ultimate principle which justifies the belief or "assumption" that the laws of nature are so rigidly inflexible and the order of nature is so absolutely uniform that "personal volition can not mingle in or interfere with the economy of nature" _is the principle of the conservation of energy_. The answer of Dr. Tyndall is now fully and clearly before our mental view, and we are prepared for the consideration of its logical validity. This answer may be conveniently divided into two propositions. First, personal volition, human or Divine, can not intermingle or in any way interfere with the economy of nature because her laws are inflexible and her order is uniform. Second, the ultimate principle which justifies the assumption that the laws of nature are absolutely inflexible and the order of nature is absolutely uniform is the principle of the conservation of energy. We shall consider this latter proposition first. There are in this proposition three ambiguous terms, which have hitherto been the source of serious misapprehension; and unless we can attain to clearer and more definite conceptions, which shall be mutually accepted, the controversy will be interminable. These are the terms "nature," "laws of nature," and "uniformity of the order of nature." We have made the attempt in a previous chapter[443] to give precision and definiteness to the concepts which these terms should connote. Referring the reader to the chapter indicated, we shall here simply restate our results. 1. _Nature_ is the aggregate or totality of all material or physical phenomena.[444] "Nature (_nascor_, to be born) means that which is produced or born."[445] 2. _A Law of Nature_ is the statement of a certain uniformity observed in the relations among phenomena.[446] The laws of nature are "simply expressions of phenomenal uniformities, having no _coercive_ power whatever."[447]