"The terms '_a priori_ truth' and 'necessary truth' ... are to be interpreted," he continues, "not in the old sense, as implying cognitions wholly independent of experiences, but as implying cognitions that have been rendered organic by immense accumulations of experiences, received partly by the individual, but mainly by all ancestral individuals whose nervous systems he inherits. But when during mental evolution, the vague ideas arising in a nervous structure imperfectly organized, are replaced by clear ideas arising in a definite nervous structure; this definite structure, molded by experience into correspondence with external phenomena, makes necessary in thought the relations answering to absolute uniformities in things. Hence, among others, the conception of the Indestructibility of Matter.... Our inability to conceive Matter becoming non-existent, is immediately consequent upon the nature of thought.... It must be added, that no experimental verification of the truth that Matter is indestructible, is possible without a tacit assumption of it. For all such verification implies weighing, and weighing implies that the matter forming the weight remains the same. In other words, the proof that certain matter dealt with in certain ways is unchanged in quantity, depends on the assumption that other matter otherwise dealt with is unchanged in quantity." In answer to the above it can be said:-- First. The current explanation of the existence of Matter is that it was created by an external agency. Mr. Spencer's lucid statement of the way in which Matter has been proved indestructible does not go far enough. Where he stops, logic might justly pronounce the whole procedure a fallacious one, a begging of the whole question at issue. The binding force of the whole argument rests upon a rational principle here overlooked by Mr. Spencer, the principle of sufficient cause. The chemist in making the experiment found that certain substances counterbalanced a given weight; after combustion, the products counterbalanced the same weight. If the weight did not change during the experiment, then no matter had been destroyed. The weight is believed not to have changed, because it existed under ordinary and quiescent conditions: which, in view of past race experience, rendered it extremely improbable that any force sufficient to vitiate the result had come into play during the experiment. _The absence of a sufficient cause to change the weight_, is, then, the critical point of the argument, and the perfect trust of the mind in the principle of sufficient cause forces us to the conclusion that Matter is indestructible.