11. Though Mr. Mansel does not acknowledge any subjective conditions of intuition besides Space and Time, he does recognize other _kinds of necessity_, which I should equally refer to Fundamental Ideas; because they are, no less than Space and Time, the foundations of universal and necessary truths in science. Such are[313] the Principle of Substance;--All Qualities exist in some subject: and the Principle of Causality;-- Every Event has its Cause. To these Principles he ascribes a "metaphysical necessity," the nature and grounds of which he analyses with great acuteness. But what I have to observe is, that whatever _differences_ may be pointed out between the _grounds_ of the necessity, in this case of _metaphysical_ necessity, and in that which Mr. Mansel calls _mathematical_ necessity which belongs to the Conditions or Ideas of Space and of Time; still, it is not the less true that the Ideas of Substance and of Cause, _do_ afford a foundation for necessary truths, and that on these truths are built Sciences. That every Change must have a Cause, with the corresponding Axioms,--that the Cause is known by the Effect, and Measured by it,--is the basis of the Science of Mechanics. That there is a Substance to which qualities belong, with the corresponding Axiom,--that we cannot create or destroy Substance, though we may alter Qualities by combining and separating Substances,--is the basis of the Science of Chemistry. And that this doctrine of the Indestructibility of Substance is a primary axiomatic truth, is certain; both because it has been universally taken for granted by men seeking for general truths; and because it is not and cannot be proved by experience[314]. So that I have here, even according to Mr. Mansel's own statement, other grounds besides Space and Time, for necessary truths in Science. 12. Besides mathematical and metaphysical necessity, Mr. Mansel recognizes also a _logical necessity_. I will not pretend to say that this kind of necessity is exactly represented by any of those Fundamental Ideas which are the basis of Science; but yet I think it will be found that this logical necessity mainly operates through the attribution of Names to things; and that a large portion of its cogency arises from these maxims,--that names must be so imposed that General Propositions shall be possible,--and so that Reasoning shall be possible. Now these maxims are really the basis of Natural History, and are so stated in the _Philosophy of the Inductive Sciences_. The former maxim is the principle of all Classification; and though we have no syllogisms in Natural History, the apparatus of _genus_, _species_, _differentia_, and the like, which was introduced in the analysis of syllogistic reasoning, is really more constantly applied in Natural History than in any other science.