At other places he introduces the idea of quality of a substance, effect of a cause, continued permanence or identity into a sensation, as necessary constituents of it; thus making a sensation a unity of complex elements instead of an isolated bare notion. How far he could have got on in his account of knowledge without this surreptitious qualifying of a professedly simple existence, may be seen by asking what would be the nature of a sensation which did not possess existence and unity, and which was not conceived as the quality of a thing or as the effect of an external reality. This digression has been introduced at this point because the next character of a sensation which Locke discusses is its objective character,--its relation to the object which produces it. To discourse of our ideas intelligibly, he says, it will be convenient to distinguish them as they are ideas in our minds and as they are modifications of matter in the bodies that cause them. In other words, he gives up all thought of considering ideas as simply mental modifications, and finds it necessary to take them in their relations to objects. Taking them in this way, he finds that they are to be divided into two classes, of which one contains those ideas that are copies and resemblances of qualities in the objects, ideas "which are really in the object, whether we take notice of them or no,"--in which case we have an idea of the thing as it is in itself; while the other class contains those which are in no way resemblances of the objects which produce them, "having no more similitude than the idea of pain and of a sword." The former are primary qualities, and are solidity, extension, figure, motion or rest, and number; while the secondary qualities are colors, smells, and tastes. The former ideas are produced by impulse of the bodies themselves, which simply effect a transference of their qualities over into the mind; while the secondary qualities are arbitrarily annexed by the power of God to the objects which excite them. It will be noticed that there are two elements which make the sensation of Locke what it is. With reference to its _production_, it is the effect which one substance, matter, has upon another substance, mind, which is unlike it in nature, and between which whatever relations exist, are thoroughly incomprehensible, so that, indeed, their connections with each other can be understood only by recourse to a _tertium quid_, an omnipotent power which can arbitrarily produce such collocations as please it. With reference to its _function_, it is the isolated and "simple" (that is, non-relational) element out of which all actual forms of knowledge are made by composition and re-arrangement.