That's why Descartes said "I think, therefore I exist". He was trying to strip away everything that could be doubted, and being conscious of the conversation he was having with himself, thought, well that conversation is something, something which I call me, and it exists -- how can I be in this conversation and simultaneously doubt that it exists -- even if the world and my body are all illusions.

Solipsism is deciding that's all there is, or in a weaker form, that that's all I'm sure of. After indulging in the doubt-fest, however, Descartes was so far from solipsism that he thought he could prove the existence of God.