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Locke is much more optimistic about our capacity to know of the existence of things than he is about our capacity to know of their nature. He presents his discussion of the knowledge of the existence of things into three parts. The first is about our knowledge of the existence of ourselves, which we know by intuition. The second is about our knowledge of the existence of God, which we know by demonstration. The third is about our knowledge of the existence of an external world, roughly resembling the world as we think it is.
We know this last category of existence by that third, pseudo-grade of knowledge: sensitive knowledge. Locke's discussion of our knowledge of the existence of ourselves and of God is almost identical to Descartes' treatment of these topics. His discussion of sensitive knowledge, however, is extremely original. Locke's mediated theory of perception raises the standard skeptical worry: If all we have access to is our ideas, how do we know there is a world out there? Locke has three strategies for dealing with this concern, and he employs all of them in Chapter 11.
Locke's first strategy, and the one he seems most viscerally drawn to, is to simply refuse to take the skeptic seriously. Can anyone really doubt, he asks, that there is an external world out there? Next, he takes a pragmatist tack. If you want to doubt that there is an external world, he says, that is just fine. All that matters is that we know enough to enable us to get around in the world. His third line of attack, however, is his most interesting. Throughout the chapter, Locke formulates a long and detailed argument based on inference to the best explanation. He presents a number of puzzling facts about our experience that can all best be explained by positing that there is an external world that is causing our ideas. Taken singly each one makes it a little more likely that there is an external world out there, but taken as a whole, Locke feels, they provide overwhelming evidence—so overwhelming that the inference is almost strong enough to be called knowledge.
Locke brings up seven marks of our experience that can best be explained by positing an external world. The first is brought up in Chapter 3, Section 14. There is a certain vivacity to veridical perception that cannot be found, say, in memories or products of the imagination. In Chapter 11, Locke offers six more empirical marks that distinguish this same set of ideas. In Section 4, he points out that we cannot get these ideas without the organ appropriate to them. No one born without the ability to hear, for example, can possibly have the idea of the sound of a French horn. Next, Locke notes that we are able to receive ideas of this sort only in certain situations. Though the organs remain constant, the possibility of experiences changes. It cannot, therefore, be the organs themselves that are responsible for producing these ideas.
In Section 5, Locke discusses the passive nature of these ideas. The next empirical mark Locke brings forth involves pleasure and pain. Some ideas, Locke claims, cannot help but be followed by pleasure of pain. When we call up the memory of these ideas, however, there is no experience of pain or pleasure accompanying them. In section seven Locke points out yet another empirical feature: a certain subset of our ideas fit into a coherent pattern so that if we have one idea, we can, with great reliability, predict another one. Finally, not only is there a predictable correlation between the ideas of taste, vision, touch, sound, etc., but there is also a correlation between the ideas belonging to different experiencing subjects (that is, between different people).
An argument based on inference to the best explanation does not add up to conclusive proof, something of which Locke is well aware. In fact, Locke seems to recognize that given his empiricism, together with his mediated theory of ideas, he can only hope to establish a strong likelihood for the existence of the external world. A certainty that precludes all skeptical doubt is, in principle, beyond his grasp.
To see why this sub-certainty is all Locke could posit based on his other theories, it is necessary to ask how certain knowledge concerning the existence of the external world could ever be attained. There are only two ways for this to be done, neither of which is available to Locke. One method would be to attempt to prove the existence of the external world a priori, through reason and innate concepts. As an empiricist, however, this argument is unavailable to Locke. Locke's epistemology is founded on the idea that all of our knowledge of the (natural) world comes to us through our experiences (the one exception he makes is for the existence of God). If one is to know, with certainty, of the existence of the external world, it must be through one's experiences.
There are two ways in which empirical knowledge comes to us. There is that which is immediately given to us through our experiences, and there is that which we infer as explanations for what is immediately given to us. The first sort of empirical knowledge, which is intuitive knowledge, can get us much closer to certainty than the second. However, since Locke has already told us that only ideas are ever presented to the mind, it is only through the second empirical means that he can arrive at any knowledge of the external world. However, arguing for an ontological claim by showing that the truth of this claim provides the best explanation for the available evidence ("the best" being always, at best, a provisional qualification) does not demonstrate the certainty of that claim, but rather its probability.
One last issue which deserves mention is Locke's pragmatic response to the skeptic. It is tempting to read this response as supporting a pragmatist understanding of truth, which says that what it means for some proposition to be true is for it to be useful and to be believed. There is some good textual evidence for this reading. At Part 4, Chapter 2.13 Locke remarks, "this certainty is as great as our happiness, or misery, beyond which, we have no concernment to know or to be." Later, at IV.xi.8, he says that our faculties, "serve us well enough, if they will but give us certain notice of those things which are convenient or inconvenient to us."
A pragmatic understanding of truth, however, runs contrary to what, elsewhere in the Essay Concerning Human Understanding, is well-entrenched realism, grounded in a vigorous correspondence notion of truth (a proposition is true if and only if it corresponds to reality). It would be odd, perhaps even incomprehensible, if Locke were here abandoning his strict realist line just to give a last response to the skeptic. It seems, therefore, far more likely that, rather than making the substantive claim that truth lies in efficacy, he is merely showing his lack of interest in skeptical concerns, or even his inability to take them seriously. He does not suggest that there may be no such a thing as the external world, but only that whether or not we can conclusively prove that there is such a world does not particularly concern him. In other words, he is stating his own unshakeable faith in realism regardless of rational proof, and adding the observation that, for all practical purposes, how we settle this issue is of no real concern.
In some sense, his claim is that the issue is strictly philosophical; it will never change the way we behave or regard the world. We will never cease to act as if there is an external world of material bodies. Even the very fact that we do not act as if we take the skeptical doubts seriously is yet another sign of how overwhelmingly probable we feel the existence of the external world to be. Despite the alleged lack of interest with which Locke regards the problem of skepticism, it seems that on the basis of what he says in the Essay Concerning Human Understanding, a very compelling anti-skeptical stance can be constructed. Even the lack of interest to which he attests can be seen as adding one more gloss to the anti-skeptical argument.