The second dialogue begins much like the first: Hylas and Philonous meet again early in the morning. Only this time Hylas is waiting for Philonous, and he has prepared himself for the discussion. He is still struggling against skepticism, and has one last recourse left: the satisfying new materialist explanation of how our sensations are caused. Modern science, he informs Philonous, tells us that sensations are caused by our brain, which is itself connected to nerves, which in turn come into contact with the outside world. In other words, objects in the outside world act on our nerve endings, which nerves then send signals to our brain, with the result being our sensations. Since this explanation is so neat and satisfying, and seems to have much evidence behind it, he thinks that it lends credence to the belief in mind-independent material objects. Philonous disagrees. If Hylas has any clear notion of a brain, he points out, then the brain must be sensible and so an idea like any other. And if it is as idea like any other, then his theory makes no senses: he would just be claiming that we have one idea which causes all the other ones. If, on the other hand, he has no such clear notion then he is speaking incoherently. So either way, the theory is a bad one. Further, no sane person would really believe that the motions of nerves actually cause sensations. This is entirely inconceivable.
Hylas admits that he is now totally sunk; he is a complete skeptic, just like Philonous predicted. He does not believe in the real existence of any sensible objects. Philonous takes the opportunity of this admission to jump in and begin taunting Hylas for his skepticism. He points to the surrounding fields, woods, groves, and streams - can Hylas really, he asks incredulously, insist that these things do not exist? Is it not absurd to say this? Hylas is taken aback once again: Philonous, after all, was the one who convinced him of skepticism in the first place, and now he is mocking him for adopting the very view he was pushing. Philonous explains that he is not a skeptic, because he did not begin with the false materialist premise, namely, that "real existence" is synonymous with "absolute existence outside of the mind". Hylas only denies that sensible objects have real existence because he understands "real existence" in this narrow way. Philonous, on the other hand, thinks that sensible objects do really exist; he thinks they really exist as ideas in the mind.
Here, Berkeley makes his most trenchant point against materialism. His last line of response against Hylas' brain-as-cause-of-sensations-theory is this: only a crazy person would really believe that motion in a physical brain could produce, say, my sensation of seeing blue. How could the one possibly cause the other? This is one mystery of materialism that we have yet to clear up: much work in current philosophy of mind is devoted to it. Despite considerable scientific progress in the fields of cognitive science as well as chemistry and physics, we are no closer today then we were in Berkeley's time of even conceiving how and why particles of matter operating on our organs give rise to the sensations that they give rise to.
Locke also recognized the puzzling explanatory gap between the physical world and our mental sensations, which are supposedly caused by this physical world. For him, this explanatory gap set the limits of knowledge. He believed that we could never understand how the motion of physical particles gives rise to mental sensations, because he thought that there really was no good reason why and how they did. In fact, this was the very spot where he felt he needed to appeal to God, dragging him into his almost-entirely materialist system of the world in order to plug up this leaky spot.
Given that this is probably materialism's weakest link, it is surprising to find Berkeley mentioning it only in passing. We might have expected to find him dwelling on this topic more, pointing out how incomprehensible mind-body interaction is in general, and touting his own system for doing away with this puzzling feature of the world.
Take a Study Break!