Many philosophers (for example Descartes) have wondered how we can know that what we think of as our waking existence is not a dream. This is slightly different from the possibility of being a brain in a vat or being deceived by an evil demon, since dreams are products of our own minds. Believing that one is living in a dream is roughly the same as solipsism (at least some kinds of solipsism), and the other possibilities mentioned require something to exist outside of the thinker.
A simpler question to ask is “should we act as though life were a dream?” A little game theory reveals that we shouldn’t. We start be considering the four possibilities and their outcomes. If our lives are a dream and we act as though we are living in a dream everything is fine. Likewise if our lives are a dream and we act as though they aren’t a dream everything is fine as well. Of course if our lives aren’t a dream, and we act as though they were the outcome is disastrous. Finally if our lives aren’t a dream and we act as through they aren’t then everything is fine. We ask ourselves then how we should act given that we don’t know if we are in a dream or not. Well given that one course of action can lead to disaster and the other can’t we should pick the sure thing, and thus should act as though life isn’t a dream.
This doesn’t answer the fundamental question however, I admit. A traditional way to answer that question of how we can know that we aren’t in a dream is to rely on some capacity (such as pain, clear thought, … whatever) that is available to us when we are awake but not when we are asleep. Although we might believe that we are awake when we are asleep we never have that missing capacity, and thus having that capacity is sure evidence that we are awake. Let us say then that our waking capacities are {A, B, C} and that our capacities while asleep are {A, B}. The missing C then is our irrefutable evidence that we are awake.
What this answer misses however is the possibility that our whole lives (that we remember at least) are the product of some dreaming state that is unlike the normal dreams that we have at night because it does provide us with C. There is also the possibility that if we ever became fully awake there would be some additional capacity, D, which we don’t have now that would serve as evidence of the state of being “really” awake. Although the traditional response does rule out the possibility that our lives are not an ordinary dream it does not exclude the possibility of “super-dreams” such as this.
I argue that we can be sure we aren’t living in a super dream because of our limitations. I don’t mean our physical capabilities, but our mental capabilities, specifically those mental tasks that we can’t but that other people can. For example consider proving an obscure theorem in topology. I probably can’t do it, even after hours of effort, even if a great deal depends on me being able to prove it. However if I open up a topology textbook I can probably find a proof of the theorem and be able to understand that proof. If my life is a dream where did that proof come from? Clearly it wasn’t part of my conscious mind. Likewise the idea that somehow I unconsciously knew the proof all along seems unlikely, since I didn’t have any “proof intuitions” helping me out.
The only conclusion that is consistent with the evidence is that there is something outside our minds, something that is able to create things such as proofs and the other mental accomplishments that we ourselves are not able to do. Yes it is true that we might be deceived in a dream into believing that we had seen a proof when we had not, but if we have the actual proof in front of us there is no denying it exists, and thus no reason to believe that we are in a “super-dream”. Of course this doesn’t rule out the possibility that our entire experience is an illusion, but it does rule out certain kinds of solipsism, which is an important first step.
On Philosophy 