Before breaking out of the Matrix, Neo's life was
not what he thought it was. It was a lie. Morpheus described it as a
"dreamworld," but unlike a dream, this world was not the creation of Neo's
mind. The truth is more sinister: the world was a creation of the
artificially intelligent computers that have taken over the Earth and have
subjugated mankind in the process. These creatures have fed Neo a
simulation that he couldn't possibly help but take as the real thing.
What's worse, it isn't clear how any of us can know with certainty that we
are not in a position similar to Neo before his "rebirth." Our ordinary
confidence in our ability to reason and our natural tendency to trust the
deliverances of our senses can both come to seem rather naive once we
confront this possibility of deception.
"And yet firmly implanted in my mind is the long-standing opinion that there is an omnipotent God who made me the kind of creature that I am. How do I know that he has not brought it about that there is no earth, no sky, no extended thing, no shape, no size, no place, while at the same time ensuring that all these things appear to me to exist just as they do now? What is more, just as I consider that others sometimes go astray in cases where they think they have the most perfect knowledge, how do I know that God has not brought it about that I too go wrong every time I add two and three or count the sides of a square, or in some even simpler matter, if that is imaginable? But perhaps God would not have allowed me to be deceived in this way, since he is said to be supremely good; [...] I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me. I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgment." (Meditations, 15)
The narrator of Descartes' Meditations concludes that none of his former opinions are safe. Such a demon could not only deceive him about his perceptions, it could conceivably cause him to go wrong when performing even the simplest acts of reasoning.
This radical worry seems inescapable. How could you possibly prove to
yourself that you are not in the kind of nightmarish situation Descartes
describes? It would seem that any argument, evidence or proof you might
put forward could easily be yet another trick played by the demon. As
ludicrous as the idea of the evil demon may sound at first, it is hard,
upon reflection, not to share Descartes' worry: for all you know, you may
well be a mere plaything of such a malevolent intelligence. More to the
point of our general discussion: for all you know, you may well be trapped
in the Matrix.
"You do not know that you are not a brain, suspended in a vat full of liquid in a laboratory, and wired to a computer which is feeding you your current experiences under the control of some ingenious technician scientist (benevolent or malevolent according to taste). For if you were such a brain, then, provided that the scientist is successful, nothing in your experience could possibly reveal that you were; for your experience is ex hypothesi identical with that of something which is not a brain in a vat. Since you have only your own experience to appeal to, and that experience is the same in either situation, nothing can reveal to you which situation is the actual one." (Introduction to Contemporary Epistemology, 10)
If you cannot know whether you are in the real world or in the word of a computer simulation, you cannot be sure that your beliefs about the world are true. And, what was even more frightening to Descartes, in this kind of scenario it seems that your ability to reason is no safer than the deliverances of the senses: the evil demon or malicious scientist could be ensuring that your reasoning is just as flawed as your perceptions.
As you have probably already guessed, there is no easy way out of this philosophical problem (or at least there is no easy philosophical way out!). Philosophers have proposed a dizzying variety of "solutions" to this kind of skepticism but, as with many philosophical problems, there is nothing close to unanimous agreement regarding how the puzzle should be solved.
Descartes' own way out of his evil demon skepticism was to first argue
that one cannot genuinely doubt the existence of oneself. He pointed out
that all thinking presupposes a thinker: even in doubting, you realize
that there must at least be a self which is doing the doubting. (Thus
Descartes' most famous line: "I think, therefore I am.") He then went on
to claim that, in addition to our innate idea of self, each of us has an
idea of God as an all-powerful, all-good, and infinite being implanted in
our minds, and that this idea could only have come from God. Since
this shows us that an all-good God does exist, we can have confidence that
he would not allow us to be so drastically deceived about the nature of
our perceptions and their relationship to reality. While Descartes'
argument for the existence of the self has been tremendously influential
and is still actively debated, few philosophers have followed him in
accepting his particular theistic solution to skepticism about the
"Instead of having just one brain in a vat, we could imagine that all human beings (perhaps all sentient beings) are brains in a vat (or nervous systems in a vat in case some beings with just nervous systems count as ‘sentient’). Of course, the evil scientist would have to be outside? or would he? Perhaps there is no evil scientist, perhaps (though this is absurd) the universe just happens to consist of automatic machinery tending a vat full of brains and nervous systems. This time let us suppose that the automatic machinery is programmed to give us all a collective hallucination, rather than a number of separate unrelated hallucinations. Thus, when I seem to myself to be talking to you, you seem to yourself to be hearing my words…. I want now to ask a question which will seem very silly and obvious (at least to some people, including some very sophisticated philosophers), but which will take us to real philosophical depths rather quickly. Suppose this whole story were actually true. Could we, if we were brains in a vat in this way, say or think that we were?" (Reason, Truth, and History, 7)
Putnam's surprising answer is that we cannot coherently think that we
are brains in vats, and so skepticism of that kind can never really get
off the ground. While it is difficult to do justice to Putnam’s ingenious
argument in a short summary, his point is roughly as follows:
Dancy, Jonathan. Introduction to Contemporary
Epistemology, Blackwell, 1985.