Locke's Substance Dilemma: Knowledge, Qualities, and the Unknowable

Locke's Empiricist Framework and the Theory of Qualities

John Locke, along with Berkeley and Hume, was a British empiricist who believed that all knowledge was based on sensory experience. In his An Essay Concerning Human Understanding (1690), Locke attempted to explain the basis of his theory in detail. However, complications arose because Locke's logically derived and systematic conclusions could not fit within a solely material and objective framework. During an era when scientific advances and evidence-based knowledge came to the fore, Locke methodically investigated the basic materials of human experience and how they produce knowledge or ideas. When he inevitably entered the realm of feeling, spirituality, and the imperceptible, his epistemological theory began showing signs of internal contradiction. Specifically, when it came to defining "substance," he confused and contradicted himself because he could neither prove it materially nor deny it logically if anything that exists was to make sense.

According to Locke's explanation, the only parts of human knowledge that objectively exist and are actually real and unchanging are qualities—specifically, primary qualities. These he listed as "bulk, figure, texture, and motion." He did not state that these were the only ones, merely the ones he presumed important. Secondary qualities were combinations of primary ones and were subject to variation from individual to individual. These included such entities as color, smell, and taste. For example, two people may agree that a certain carpet is flat and square but may vary in their opinions of its softness and the quality of its blue thread, depending on visual and tactile acuity and personal judgment.

Simple and Complex Ideas: Building Knowledge from Experience

Locke postulated that together, primary and secondary qualities stimulated the mind through sensory experience to produce simple ideas, such as "the color red." These simple ideas become other simple ideas and through reflection may even become complex thought. Complex ideas are combinations of simple ideas. Simple ideas can be compared, relationships identified, or they can be sorted into categories according to similarity. This last way of forming complex ideas is "abstraction," and this is what we are doing when we give a name to a substance, like "apple" or "gold." Thus, Locke argues there is no such thing as "innate knowledge" or original thought.

Simple ideas cannot be broken down into simpler forms and include a uniform perception such as that of "redness." This redness does not necessarily have a given form—it simply is. It cannot be perceived objectively because redness does not exist without some form or substance in the material world, at least as far as we can know. According to Locke, qualities "cannot be imagined to [i.e., it is inconceivable that they] subsist by themselves." The substratum wherein these qualities were perceived to exist was defined as "substance." The difficulty was in proving objectively what exactly this "substance" was, though all logical thought pointed to the fact that it must exist.

The Logical Necessity and Perceptual Impossibility of Substance

However, Locke's own theory—stating that all knowledge was based on experience and thus must first be perceived by the senses—contradicted the characteristic of "substance" being unperceived. Thus, if Locke was right, substance must also logically not exist. This is the fundamental dilemma that Locke could not resolve.

The problem with the concept of "substance" is that it eludes explanation through words and slips through the mind's fingers just when one thinks he has grasped it. Locke calls the idea of substance "obscure," yet insists on using it, stating that it must exist, though we will never be able to define or prove it. To any scholar of knowledge, such a conclusion would be unacceptable, but in his confusion, Locke apparently did not delve further—perhaps in fear of what he might find.

He thinks the idea of "pure substance" is an empty one. On one hand, he concludes it is impossible for qualities to exist out there without something having them, thus the necessity of the idea of "substance." On the other hand, any definition and description of such a concept would by necessity have to be in terms comprehensible to the human mind. As such, they must be based on experience through sensation and reflection. Thus, "pure substance" must be defined in terms of primary qualities, which paradoxically are themselves explained in terms of it. We are speaking, says Locke, like little children when we talk about pure substance: "If anyone will examine himself concerning his notion of pure substance in general, he will find he has no other idea of it at all, but only a supposition of he knows not what support of such qualities, which are capable of producing simple ideas in us."