Suppose a mad scientist removed your brain and placed it into a vat of nutritious fluid which kept it alive. The nerve endings of your brain are then connected to a super-computer which gives you the illusion that everything is normal. In fact, this illusion is flawless. You can tell no differences between reality and the simulated reality. You can see trees, flowers, houses, and any other object that you can see in reality. The only difference is that in reality these objects are only electric stimulus provided by the super-computer.

Imagine now that you always lived in a vat. How would you know you weren’t in this predicament? Hilary Putnam tries to answer this question in his book Reason, Truth and History. I am only going to be giving a brief summary of his thoughts, but I would encourage you to read the entire chapter.

Let us suppose that we are actually brains in a vat. Would it be possible for us to say or think that we were brains in a vat? According to Putnam, the answer is no. The reason for this is related to the causal theory of reference. When I say “there is a tree in front of me.”, there is no special power in the words themselves. The words are actually referring to objects (in this case a tree) that actually exist. However, if you are a brain in a vat, when you say “there is a tree in front of me”, you are referring to electric stimulus provided by the super-computer and not a “real” tree. In matter of fact, there is no way for you to reference a “real” tree. There is no thought or word that references a “real” tree in vat-english[1]. So this means when you say “I am a brain in a vat” in vat-english, you are referring to an electric stimulus and not a “real” vat and brain. If this is a case, the statement “I am a brain in a vat” will always be false. If you were a brain in a vat, this sentence would be referring to a vat-english brain and vat which would make the statement false. Likewise, if you weren’t a brain in a vat, the sentence would also be false.

So according to Putnam, the statement “I am not a brain in a vat” will always be true. However, if we extend this idea a little farther, we can discover some interesting consequences. Let us assume that you are a brain in a vat and all that you know is the simulated reality provided by a super-computer. When you say that “I am not a brain in a vat”, you would be correct in saying it. One day though, the mad scientist decides that he wants to hook you up to your old body. At that moment, you would see reality through “real” eyes. You would see a huge table filled with vats containing the brains of all your friends. And most importantly, you would be able to comprehend that a moment ago your brain was in a vat. However, after a couple hours of freedom, the mad scientist got bored and put your brain back into the vat.

The interesting point is that you would now be able to correctly say that “I am a brain in a vat”. Several hours ago this statement would have been false. How can this be? The words are exactly the same. The reason for this is that each statement is referring to two different objects. Before your Matrix-like experience, you were referring to the vat-english brain and vat. However, after the experience, you are now referring to the real-english[2] brain and vat. In addition, the validity of these two statements are independent of time. For example, if you found a friend (who’s brain is also in a vat) after your out-of-vat experience and tried to tell him about your experience, he would think you were crazy. He would try to tell you that your brain is not in a vat… and he would be correct. However, you now think you are the “chosen-one” and you INSIST that your brain is in a vat… which also happens to be correct. At this moment, you might give up, throw your hands in the air, and conclude that everything is relative. The crazy part is that everything is not relative. These two people are referring to two different things. It is like comparing apples to oranges. One is speaking vat-english and the other real-english. So the idea of absolute truth is not violated in accepting both of these statements.

Imagine that this argument with your friend continues. Out of a fit of frustration, you say that it is like my brain is in a vat (in the vat-english definition of the word). Although your friend would probably still disagree with you, he would be wrong. The reason for this is that instead of making a direct reference to brains in a vat (in real-english), you are actually making a comparison. Since real brains and vats are very similar to virtual brains and vats, the above statement would be true. In fact, when a person says that his brain is in a vat (in vat-english), I don’t think he ever intended to refer to vat-english brains and vats. Instead, he likely meant that it is like his brain is in a vat. Since it is impossible for the person to directly reference an external object, he will have to settle for a loose comparison. However, this is not to say that someone else or sometime in the future he may be able to directly reference this external object.

[1] vat-english is a language whose words refer to objects in a illusion created by the super-computer.
[2] real-english is a language whose words refer to objects in physical world.