Back to Rene Descartes for a moment, here's some history.
When Descartes decided to prove that the world as we know it does indeed exist, he started with The Method of Doubt (showing why we might think that things are not as they seem); he went on with Cogito, Ergo Sum (showing that, at the very least, he did exist); and then by adding some faith in God into the mix, Descartes decided that he could be pretty sure that the basic truths (math, logic) are indeed true. But up to this point he has been working with the mind, and the material world is not all in your head. So all this stuff doesn't apply to sensory input, does it?
We would like to think that it does. But how could he be sure? Well, he came up with two arguments that seemed to prove that the mind is separate from the body.
Argument #1: I (my mind) is not dubitable (in doubt). But here I am, doubting my body. So my body is dubitable. And if two things do not share the same intrinsic properties, they cannot be the same thing.
But we can argue that... dubitableness is not an intrinsic property. You have to have a doubter to doubt, and as such, the property does not exist on its own, apart from the rest of the universe. Doubt is, in fact, an extrinsic property.
So... Argument #2: My mind is not divisible. But my body is (by cutting off a finger, for example). They have different intrinsic properties, so they are indeed different things.
But... I'm not sure how well this went over in Descartes time, but it's not accepted today. The mind does seem to consist of different 'parts'; ask any neurologist or psychologist. Of course, the brain is not the same thing as the mind, but the mind can indeed be lost 'in parts'; you can loose some memory, or some senses, or some rationality, etc.
Of course, the alternative, that mind is nothing more than a feeling that mysteriously arises from the functioning of the brain, is not accepted by everyone even today. All this historical review shows us is that Descartes, as was so often the case, falls short of the stringency currently expected in the field of philosophy. However, this does explain where certain common arguments and viewpoints first appeared. Needless to say, the mind/body problem is still being debated today, and there is no end in sight.