The classic sceptical challenge is - "How do you know you are not just a brain in a vat?" This challenge is meant to capture the epistemological impact of the fact that there is no means of knowing with any certainty that we are not just a "brain in a vat", or one of the victims wired into a pod in "The Matrix", or victims of Descartes' Demon. The callenge to the realist / materialist is that all of our evidence for and about Reality arrives via our senses. And there is no way to determine with any certainty that our senses are not being systematically fooled.
To this challenge, I offer a the following responses:
1. The concepts "reality" and "dream [world]" refer to two distinctly different modes of experiencing "The slings and arrows of outrageous fortune". When experiencing life in one mode, we notice that things perceived are constant, persistent, consistent, and coherent. When experiencing life in the other mode, we notice that things perceived are dramatically less constant in form and character, often transient in existence, frequently mutually inconsistent both from thing to thing and across time, and far more frequently quite incoherent. One mode of experience draws the focus of our attention, is amenable to inquiry, and responsive to our reactions. The other mode of experience often drifts uncontrollably past our attention, is rarely subject to inquiry, and is often unresponsive to our reactions. On any scale of measure, the difference between the two modes of experience is dramatic and unmistakable whenever noticed. One of these modes of experience we call the "real word", the other we call the "dream world" (or hallucinations, or illusions, or "The Matrix" world).
Most of us spend most of our time experiencing life in the "real world" mode. Episodes spent in the "dream world", while they may seem quite real at the time, always end with a transition back to the "real world" mode of experience. Some people, for reasons as diverse as drugs to organic brain damage, spend more of their time in the "dream world". Some people, again for diverse reasons, lose the ability to notice the distinctly different character of two modes of experience, and are unable to distinguish their "real" experiences from their "dream" experiences. But both of these sorts of people are classed as "not normal" or "insane". The bottom line is that life is not a dream. The "real world", unlike the "dream world" possesses an unmistakably greater degree of constancy, consistency, and coherence. In the real world, elephants are huge, grey and don't fly. That remains true across time, and is consistent with all other information we have about the real world mode of experience. In the dream world, pink elephants can buzz around your head, and turn into green mice stomping on the roof of your house. The fact that sometimes a dream appears so real you can't tell, does not alter the fact that you always wake up.
2. Yet the "brain in a vat" scenario presupposes that the "reality" being fed into my brain through all those electrodes (or whatever) is cognitively indistinguishable from a "real reality". And that implies that the "fake reality" being created for me is judged (or remembered) by me to be constant, persistent, consistent, and coherent, drawing the focus of my attention, amenable to my inquiry, and responsive to my reactions. If we assume, for the moment, that my memories of experiencing such a reality are not also fake, then the generators of such a "fake reality" face an enormous problem. That problem is one of "combinatorial explosion". From moment to moment, I appear to be free to turn my focus of attention to any spot, make almost any inquiry, and test the responsiveness of almost any aspect of the reality I am experiencing. So as moment follows moment, the possibilities that the "fake reality" generators must maintain as constant, persistent, consistent, and coherent explodes exponentially. Have you ever tried to tell a lie in a complex dynamic situation? Pretty soon it becomes impossible to keep the web of lies consistent. The easiest and simplest way to do it, is to base the entire web of lies on some reality. In order to overcome the difficulties of maintaining consistency across an exponentially exploding series of "little lies", the "fake reality" generator must be basing its inputs to me on a form of "real reality" that is itself constant, persistent, consistent, and coherent, amenable to inquiry, and responsive to my reactions. Which would mean that being a "brain in a vat" would make no practical difference. What I am experiencing is most probably a "real reality" somewhere, if not necessarily where I expect.
3. Whether I am a "brain in a vat" or the victim of "Descartes' Demon", there is a reality (a mode of experience that I discern to be constant, persistent, consistent, and coherent, draws the focus of my attention, is amenable to my inquiry, and responsive to my reactions) that I am experiencing, to which I must react properly. In simple terms, if I do not accurately predict which way the tiger will jump, I will end up being dinner instead of having dinner. And it matters not whether the "Tiger" I am experiencing is "fake" in some sense or not.
There is no shred of evidence in my experience, or in my memory that would justify a belief that I am a "brain in a vat". To the best of my determination, it makes no practical difference to the way in which I need to react to my experiences of this reality I experience. So I choose to employ Ockham's Razor and reject the hypothesis that this "reality" I experience is not in fact the "real" Reality. Does it remain possible that I am, in fact, a "brain in a vat"? Yes, of course. But to all appearances, it seems to make no discernible difference. And a difference that makes no difference, is no difference. The necessary response to the sceptic, therefore, is "So what? Who cares? What difference could it possibly make?"
To the Sceptic I would therefore advise: if you are worried you might be wired into a pod in The Matrix world, make a few random investigations and verify that your reality stays constant, persistent, consistent, and coherent, amenable to inquiry, and responsive to your reactions. If it does, then relax and enjoy the trip. And keep in mind that if your reality is responsive to your reactions, you will have to accurately predict which way the tiger will jump, lest you become dinner rather than enjoying it. Even the plot line of "The Matrix" cannot avoid that necessary consequence of the consistency of "reality" (according to the story-line, the wired-in subject will die in reality if he is killed in the virtual world).