Locke, as an empiricist, cannot infer the existence of mind-independent material objects from innate concepts; he must infer them from his sensory experience. In fact, though, Locke has three strategies for dealing with this concern, and he employs all of them in chapter xi of the Essay Concerning Human Understanding. 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? Another strategy he makes use of is to give a pragmatic response. 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.
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 that there is a certain vivacity to perception that cannot be found, say, in memories or the products of the imagination. Berkeley too, as we will see, makes use of this mark of sensations. In chapter XI Locke offers six more empirical marks which distinguish this same set of ideas. He points our that we cannot obtain these ideas if we do not have 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 points out that we are able to receive ideas of this sort only in certain situations. Though the organs remain constant, the possibility of experiences change. It cannot, therefore, be the organs themselves that are responsible for producing these ideas. In section five Locke discusses the passive nature of these ideas; they are not voluntary, but involuntary, coming to us unbidden and unavoidably. We cannot just choose to have the experience of tasting watermelon at will, for example. Nor can we choose to avoid hearing the blaring siren of a car alarm at four o'clock in the morning.
The next empirical mark Locke brings forth involves pleasure and pain. Some ideas, Locke claims, cannot help but be followed by pleasure or pain. For example, when we have the sensation of seeing our flesh cut with a knife, this will almost certainly be accompanied by a sensation of searing pain (unless we are heavily medicated). 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. The knife and pain example above can serve to illustrate this point as well. Another example of this mark of experience would be the fact that our sensation of seeing a hand let go of a book in midair is always followed by a sensation of seeing the book fall. 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).
No single one of these marks proves conclusively that our experiences are caused by mind-independent material objects. However, as Locke points out, all of these marks, singly and as a group, can be coherently and compellingly explained by positing that our experiences are caused by mind-independent material objects. This makes that hypothesis overwhelmingly plausible — so much so, that it would be unreasonable for us to doubt it.
Berkeley never considers the possibility of proving the existence of mind- independent material objects by inference to the best explanation, but it is easy enough to guess what he would say about this line of reasoning. He would claim that his own idealist hypothesis explains all the evidence just as well as the materialist hypothesis. Each of those marks of experience, both singly and as a group, can be easily accounted for on his theory.
How could Locke then respond to Berkeley? He could counter that Berkeley's hypothesis does not explain the evidence just as well as the materialist hypothesis. For one thing, we tend to think that an explanation is better if it is simpler. But Berkeley's explanation is needlessly complex: where Locke just needs there to be objects out in the world, Berkeley needs there to be both God and the ideas he is causing us to have. In addition, where Locke just needs us to passively perceive the objects in order to have our experiences, Berkeley needs to tell a complicated story (as we will see) about how God shows us the ideas in his mind, and when He does this, and why. Furthermore, Berkeley does not even really succeed in accounting for all of the marks of experience. He never really explains, for instance, why our sensations always follow certain patterns, other than to assert that they follow these patterns because God shows them to us in these patterns. But why, we could press him, does God show us ideas in these patterns? Certainly, he is not bound by any physical necessity. Locke, the materialist, has a ready and satisfying explanation for why our sensations follow certain patterns: these are the patterns which the objects themselves are governed by, necessitated by physical law.