John Locke thought of himself as a realist (not in the Medieval sense of accepting the reality of universals, but in the modern sense of believing in a mind-independent world). Yet it seems that his philosophy leaves no room for any knowledge of that alleged world, as Berkeley and Hume pointed out. Locke believed that all knowledge comes by means of sense experience (thus he is an empiricist, as opposed to being a rationalist such as Descartes–it is ironic that in his hierarchical classification of knowledge Locke lists intuitive knowledge as first, demonstrative knowledge as second, and sensory knowledge as the lowest form of knowledge, barely to be called knowledge). Locke believes that knowledge arises by means of ideas in the mind. Whether these ideas are images or something else remains a subject of debate among Lockean scholars. In any case, Locke believes that that a quality is the power to produce an idea in the mind. Primary qualities are actually in the thing-in-itself, and our ideas of primary qualities are isomorphic with the actual structure of the physical substance we perceive. Primary qualities are measurable, and include size, shape, and mass. Secondary qualities are not in the thing itself; our ideas of secondary qualities are not isomorphic with the actual structure of the material substance. However, the primary qualities interact with human sensory organs and with the human brain to produce ideas of particular colors, odors, sounds, and tastes. Thus, secondary qualities have a partial basis in the thing-in-itself despite the lack of isomorphism between idea and thing.
The classic problem with this view is that Locke claims that we are only aware of our own ideas. We do not have any direct access to the material substance, to the thing-in-itself. In fact, substance is just that which underlies the qualities, a “something-I-know-not-what.” But if we lack access to the thing-in-itself, there is no way to compare our ideas to the actual object allegedly causing those ideas to determine which qualities are primary and which ones are secondary. Access to knowledge of extramental reality seems impossible, and a trip down the phenomenalist brick road of Berkeley, Hume, and the sense data theorists of the early twentieth century. Such an idealistic journey is not what Locke wanted to make. Idealism has serious difficulties; the source of the ideas (our own minds? the mind of God) remains a mystery, and the orderly nature of the phenomena we experience is left unexplained unless a person takes the Berkeleian route of positing God to explain natural laws. Direct realism is another option; the label of “naive realism” is a pejorative and is a blatant attempt to beg the question regarding the truth or falsity of direct realism. As for the straw men critics of direct realism try to knock down, no direct realist has denied the possibility of illusion. It is Berkeley and Hume’s phenomenalism that cannot distinguish between illusion and reality except by taking Hume’s route of more vivid ideas (which he calls impressions) being the most “real.”
Aristotle and St. Thomas Aquinas were both direct realists. Aquinas accepted the idea that knowledge comes through the “phantasm,” or sensory image, from which the mind extracts the intelligible content from a material substance. Thomists today often say that the difference from Locke’s view is that Locke believed we have access to ideas, not the thing in itself–it is the ideas that we know. In contrast, Aquinas believes that it is through the phantasm that a person gains some knowledge, albeit limited, of the thing-in-itself. But does this really avoid Locke’s problem or does it evade it by a kind of word game?
After reading more of how contemporary Thomists deal with the epistemological gap, I must back away from my earlier position that Thomism does not avoid an epistemological gap between mind and thing. Contemporary Thomists believe that humans have evolved as part of their environment, not as creatures separate from their environment. Even thought knowledge is of “external” things, there is a communication of intelligible content from object to subject–agent causation is not limited to human agents. The phantasm contains the information that human beings extract to help them to live in the environment in which they are embedded, to the point that the person becomes “intentionally one” with the thing-in-itself. While Duns Scotus posited intuitive knowledge of an object as existing in addition to a rather traditional Aristotelian account of knowledge, I am not sure that such an intuitive knowledge is necessary for human beings to get by in the world. If such intuitive knowledge exists (perhaps in the form of psi), such knowledge could speed up our apprehension of a thing and determine whether or not it is dangerous. But if the mind is not considered a container, but as one way of an organism’s acting in the world, that seems to eliminate the Lockean gap between idea and thing. The phantasm becomes that “by which” a person apprehends some aspects of the being of a thing.