68. The relation of cause and effect, according to Locke’s general statement as to relation, must be something ‘not contained in the real existence of things, but extraneous and superinduced.’ (Book II. chap. xxv. sec. 8.) It is a ‘complex idea,’ not belonging to things as they are in themselves, which the mind makes by its own act. (Book II. chap xii. secs. 1, 7, and chap. xxv. sec. 1.) Its origin, however, is thus described:--‘In the notice that our senses take of the constant vicissitude of things, we cannot but observe that several particular, both qualities and substances, begin to exist; and that they receive this their existence from the due application and operation of some other being. From this observation we get our ideas of cause and effect. That which produces any simple or complex idea we denote by the general name cause; and that which is produced, effect. Thus, finding that in that substance which we call wax, fluidity, which is a simple idea that was not in it before, is constantly produced by the application of a certain degree of heat, we call the simple idea of heat, in relation to fluidity in wax, the cause of it, and fluidity the effect. So, also, finding that the substance, wood, which is a certain collection of simple ideas so-called, by the application of fire is turned into another substance called ashes, _i.e._ another complex idea, consisting of a collection of simple ideas, quite different from that complex idea which we call wood; we consider fire, in relation to ashes, as cause, and the ashes as effect.’ Here we find that the ‘given,’ upon which the relation of cause and effect is ‘superinduced’ or from which the ‘idea of it is got’ (to give Locke the benefit of both expressions), professedly, according to the first sentence of the passage quoted, involves the complex or derived idea of substance. The sentence, indeed, is a remarkable instance of the double refraction which arises from redundant phraseology. Our senses are supposed to ‘take notice of a constant vicissitude of things,’ or substances. Thereupon we observe, what is necessarily implied in this vicissitude, a beginning of existence in substances or their qualities, ‘received from the due application or operation of some other being.’ Thereupon we infer, what is simply another name for existence thus given and received, a relation of cause and effect. Thus not only does the _datum_ of the process of ‘invention’ in question, _i.e._ the observation of change in a thing, involve a _derived_ idea, but a derived idea which presupposes just this process of invention. Rationale of this ‘petitio principii’.