How do I know I'm not a brain in a vat, with all my sensory experiences being generated by a supercomputer?
You will of course already be familiar with the idea of a virtual reality: the kind of reality generated by computers in flight simulators, computer games, and so on. The objects that one sees on the computer screen aren't real, they are merely "virtual". They are a product of the computer programme.
The brain, it seems, is vital so far as consciousness is concerned. And your brain is connected to the rest of your body via your nervous system. When, for example, you perceive a tree in front of you, the light rays from the tree are refracted into the eye where they fire tiny light-sensitive cells, As your view changes, so the patterns of firings produced by these cells alters accordingly. The light sensitive cells are connected via the optic nerve to the brain. Severing this pathway between eye and brain results in blindness.
Now consider the following rather horrible story. Suppose that last night, Martians flew down in their flying saucer and broke into your house. The Martian's drugged you and then surgically removed your brain from your skull.
They then flew your brain back to Mars (being careful to keep it alive). There they installed your brain in a vat of life-supporting liquid, and wired it up to an immensely powerful super-computer.
This computer is designed to run a virtual reality programme. It is designed to stimulate your brain in just the same way it would be stimulated if you still had a body and were still back on Earth. So when you woke this morning, the computer produced exactly those patterns of neural stimulation that your body would have produced were you still to have a body and be in your bedroom. So you believed that you still had a body, and that you were still in your bedroom. Right now, you think you are reading these words on a computer screen back on Earth. But in fact you are a brain in a vat on Mars, and the world you think you see around you isn't real: its merely a virtual reality.
Or perhaps you have been a brain in a vat since birth. Perhaps your entire life has been lived out within a virtual reality. Your friends and family, your house and street, all the things that seem so familiar to you, don't really exist. They are no more real than are the characters and objects in some computer game. They are merely virtual characters and objects.
Now the problem this story raises, of course is: how do you know that you aren't a brain in a vat? For everything would seem just the same to you even if you were. Perhaps you aren't a brain in a vat. Even if you aren't, it seems you can't know you aren't, for whilst you may believe you aren't, you have no justification for believing that you aren't, and a belief requires justification in order to count as knowledge.
So it seems you don't know that you are not a brain in a vat, that you are on Earth, that you have a body, etc.
On the other hand, it seems just silly to deny that you know these things. I want to say: of course I know that there is a desk in front of me, that I have fingers that I am typing on a computer keyboard, and so on. That's just "common sense", surely?
But "common sense" has been wrong before. It was once the "common sense" view that the Earth is flat. "Common sense" was wrong about that. Maybe "common sense" is wrong about our having knowledge of the external world too?
The problem of deciding between the "common sense" view and the conclusion of the brain-in-a-vat argument is one with which you will be presented in your first year. Philosophers are divided over which view is correct. Some philosophers - those who are "sceptics" about the external world, argue that we can't know about the external world. Other philosphers maintain that there is something wrong with the sceptic's argument. The problem, of course, is in finding what is wrong with it. What do you think?
For further reading on this problem, try Nigel Warburton's Philosophy: The Basics (mentioned earlier), chapter 4.
Can I step into the same river twice?
Suppose someone were to claim that they have discovered a profound philosophical truth: that you cannot step in the same river twice. They explain as follows. If you step into a river, get out, then step back into it again, a certain amount of water will have flowed by in the meantime.
Here's a similar argument. You cannot meet the same person twice. For when you meet a person a second time, they will have changed in various ways. They will have new memories, for example. So it won't be the same person. Therefore there must be two people you meet: the person you meet first, and a second, distinct person that you meet the second time. So we have discovered two profound philosophical truths. Or have we? For of course, it sounds quite mad to deny that you can't step into one and the same river twice. Of course you can! That's just "common sense"! But perhaps we should give up "common sense" at this point? Perhaps that is what these arguments show?
In fact, both arguments involve a confusion. Both arguments trade on an ambiguity in the use of the word "same". There are (at least) two senses of "same". There is qualitative sameness, and then there is numerical sameness. Consider two bowling balls, both of which weight the same, both of which are black, both of which share exactly the same molecular and sub-molecular make-up.
These two balls are qualitatively the same: they share the same qualities. But they aren't numerically one and the same ball: there are two balls not one. So there can be qualitative sameness without numerical sameness. Now suppose we take one ball and paint it white.
Now its qualities have changed. It is not qualitatively the same as it was before. But it is numerically one and the same ball. So there can be numerical sameness without qualitative sameness.
Now consider the sentence:
"You cannot step into the same river twice"
What sort of sameness are we talking about here? It isn't clear. If the claim is that you can't step into qualitatively the same river twice, then the claim is obviously true but hardly philosophically profound. If, on the other hand, the claim is that you can't step into numerically the same river twice, then the claim, whilst philosophically more intriguing, is pretty obviously false.
Now it is the second, more philosophically intriguing claim that the argument is supposed to support. But as should now be clear, it is a bad argument. The conclusion that you can't step into numerically the same river twice doesn't follow from the premise that you can't step into qualitatively the same river twice. The argument trades on an ambiguity in the use of the word "same".
The moral is: sometimes seemingly profound philosophical insights are merely a product of linguistic confusion. And a clear value in studying for a Philosophy Degree is that it enables you to tell the wheat from the chaff.