“The French philosopher Descartes, in an effort to make proof doubly sure, demanded that we prove the existence of things outside the mind. We might be dreaming, he said, when we think we know the world of physical things, and there is no sure way of knowing the difference between the waking state and the state of dreaming.
Modern philosophy has in general followed Descartes on this point, demanding that we start from inside our own mind and prove both the existence of the world and of other human beings. This position is called subjectivism because it is based on the consciousness of the thinking subject, making the object of knowledge a part of the thinking subject himself, his ideas, feelings and so forth, so that there is no objective, external test of truth.
Descartes, when he demanded proof for the existence of the outside world, started a false problem which gave rise in modern philosophy to innumerable errors: a false problem because the question is asked in such a way that no answer is possible, as though we were to say, ‘Prove that Julius Caesar was the third President of the United States.’ The fact of the existence of the outside world is not an abstract truth. It is not a necessary truth, for any number of possible worlds other than our own is conceivable. The existence of the world of bodies of which we are a part is no more necessary than my existence or yours. It could have not been, just as we might have not been. That it exists at all is something that we discover, not prove.
Let us take a further look at the point, for it is of crucial importance.
All the beings we know by direct experience, including ourselves, are contingent existences; that is, they need not have existed, and they can go out of existence. To know that they exist in fact is to experience that existence directly, here and now. To know that the wall is brown, for example, I have to sense it immediately. I cannot take it on faith. If I take your word for it, a painter may be changing its color to green while you are telling me that it is brown. Similarly it cannot be proved syllogistically. In the very act of sating the syllogism that leads to this conclusion, the color of the room could be changed. We do not invent, or create the existence which form the field of our knowledge. We discover these existences, and there is no possible way of knowing what has, in fact, been given existence other than to discover it (many other kinds of existences than the ones we know could have been brought into being.)
For the philosopher to ask proof of the actual existence of contingent things, including his own existence, is to betray the evidence of the fundamental intuition of his senses and intellect. It is to ask proof for what does not need proof, for what indeed cannot be proved, since it is prior to proof and is implied in all demonstration.”
– Daniel J. Sullivan, An Introduction to Philosophy –
– Lucas G. Westman