Sensory experience is not capable of giving rise to an idea of substance. For Locke, the senses convey information, which he calls simple ideas. The simple ideas are in part the qualities of objects, such as color, taste, feel, etc. We can infer from these simple ideas or the ordinary qualities observable in objects to the idea of substance. Observing that several of these simple ideas go together we can assume that there is some substratum wherein these simple ideas subsist, that is, substance. “These, and the like fashions of speaking, intimate, that the substance is supposed always some thing besides the extension, figure, solidity, motion, thinking, or other observable ideas, though we know not what it is.” Substance is the some-thing we know not what that causes sensation.
The experiential concept of a cherry includes its visually perceived redness, the kinesthetically felt smoothness of its exterior, and the sweetness of the cherry’s taste. Moreover, the cherry has a certain mass, is extended in space, and is dense enough to be solid in most cases. These all constitute the experiential concept of a cherry. To this the mind adds an idea of substance, which ultimately gives rise to these sensible qualities. Locke writes, “no true conception of any modes or accidents… [can] exist or subsist of themselves… Hence the mind perceives their necessary connexion with inherence or being supported; which being a relative idea superadded to the red colour in a cherry, or to thinking in a man, the mind frames the correlative idea of a support.” The correlative idea that supports what is perceived is substance. Substance constitutes the object of sensitive knowledge.
Locke claims neither that we have an idea of substance nor that we lack any idea of substance. We neither have an innate idea of substance nor lack an idea of substance in the strict sense. By strict sense, I mean that we can identify exactly what substance is or is not. On the one hand, Locke, a rabid empiricist, believes that the idea of substance arises from experience. Experience allows us to form simple ideas. When we abstract by combining several simple ideas, we realize that substance underlies all things. If this were not so, then we would have to accept that modes or accidents subsist by themselves. Locke writes, “It is by such combinations of simple ideas, and nothing else, that we represent particular sorts of substances to ourselves.” We are not endowed with the idea of substance a priori. Only after having experienced something do we realize that substance supports the properties of objects. Sensible qualities, according to Locke, cannot subsist alone, so “we suppose them existing in and supported by some common subject” – substance. On the other hand, we do not completely lack any idea of substance. We are capable of knowing that substance exists, even though we are not able to understand what substance is. Locke writes, “we have no clear and distinct idea of the substance of matter.” Sensible qualities exist in and are supported by substance.
Locke, in a sense, does ridicule those who use such jargon. The idea of substance for Locke comes to the same conclusion as one who accepts innate ideas. Locke believes that a substratum does exist, something we know not what, but we do not possess such a concept a priori; rather, only through experience do we know of substance since it is not possible for simple ideas to subsist alone. Locke’s concept of substance may come to the same conclusion as one who accepts innate ideas, but he offers a different means to grasp the idea of substance. So, it seems that the texts of the Essay does present a coherent account, even though some objections may be raised against Locke’s account and whether he is true to empiricism.
Locke’s remarks in the texts to Stillingfleet do seem consistent with those in the Essay. The context is different since Locke is defending himself against an objector who believes that the innate idea of substance is possible. Locke is adamant that substance gives rise to sensitive qualities. He says, “that the general idea we have of substance is, that it is a substratum or support to modes or accidents, wherein they do subsist.” He does make some curious remarks. For instance he says, “the mind forms it [substance], because it cannot conceive how they should subsist of themselves.” How does the mind form the idea of substance? Is Locke retreating from his original position? If the mind forms the idea of substance, then is Locke suggesting a more rationalistic position than he originally supposed? In the texts to Stillingfleet, although Locke tries to be consistent (and is so on the surface), he does seem to be retreating from his original stance offering some reason to acknowledge innate ideas as possible.