How now does the understanding become possessed of ideas? Only through experience, upon which all knowledge rests, and on which as its principle all knowledge depends. Experience itself is twofold; either it arises through the perception of external objects by means of the sense, in which case we call it sensation; or it is a perception of the activities of our own understanding, in which case it is named the inner sense, or, better, reflection. Sensation and reflection give to the understanding all its ideas; they are the windows through which alone the light of ideas falls upon the naturally dark space of the mind; external objects furnish us with the ideas of sensible qualities, and the inner object, which is the understanding itself, offers us the ideas of its own activities. To show the derivation and to give an explanation of all the ideas derived from both is the problem of the Lockian philosophy. For this end Locke divides ideas (representations or notions) into _simple_ and _compound_. _Simple ideas_, he names those which are impressed from without upon the understanding while it remains wholly passive, just as the images of certain objects are represented in a mirror. These simple ideas are _partly_ such as come to the understanding through an individual sense, _e. g._ the ideas of color, which are furnished to the mind through the eye, or those of sound, which come to it through the ear, or those of solidity or impenetrability, which we receive through the touch; _partly_ such as a number of senses have combined to give us, as those of space and of motion, of which we become conscious by means of the sense both of touch and of sight; _partly_ such as we receive through reflection, as the idea of thought and of will; and _partly_, in fine, such as arise from both sensation and reflection combined, _e. g._ power, unity, &c. These simple ideas form the material, as it were the letters of all our knowledge. But now as language arises from a manifold combination of letters, syllables and words, so the understanding forms complex ideas by the manifold combination of simple ideas with each other. The complex ideas may be referred to three classes, viz.: the ideas of mode, of substance, and of relation. Under the ideas of mode, Locke considers the modifications of space (as distance, measurement, immensity, surface, figure, &c.), of time (as succession, duration, eternity), of thought (perception, memory, abstraction, &c.), of number, power, &c. Special attention is given by Locke to the conception of substance. He explains the origin of this conception in this way, viz.: we find both in sensation and reflection, that a certain number of simple ideas seem often to be connected together. But as we cannot divest ourselves of the impression that these simple ideas have not been produced through themselves, we are accustomed to furnish them with a ground in some existing substratum, which we indicate with the word substance.