I think, therefore I am. (in Latin)
When Rene Descartes reached middle age, he got the idea that he should list all the things that he could be absolutely certain of. He discovered that there wasn't much -- he couldn't trust his eyes (he might be dreaming), or even mathematics (he could conceive that his mind was fooling him even about simple arithmetic). Using The Method of Doubt, he narrowed it down to just one certainty: I think, therefore I am. He decided that it was impossible that he was not thinking, as even the doubt that he was thinking was a thought. And if his thoughts existed, he must also exist. This was quite a relief.
Descartes didn't stop there. He believed that some things were necessary for him to think. For example, God. And as God is omnipotent and benevolent, he would not have us deceived (at least, not completely). So, he says, we can trust such things as logical and mathematical truths, and probably our eyes and ears. But perhaps we still cannot trust our senses. After all, we are still working in the realm of the mind at this point, not matter. And after all, the mind is different than the body, isn't it...?
And so on to The Mind/Body Problem.