In 1,637, French philosopher René Descartes recorded a basic truth in his grandly entitled Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences:

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Je pense, donc je suis.

“I think, therefore I am.” It was the one fact that he could not doubt, something he was so certain of that he made it his first principle of philosophy. You will no doubt be certain of this fact, too: you feel; you experience joy; you see, smell and taste the wild and wonderful technicolor world around you; you think, therefore you exist. But do I? Of course, like Descartes, I know my own fundamental truth that I exist, but I don’t know for sure that you exist. Our conscious experience of reality is wholly personal, completely subjective, and many philosophers argue that the only principle without doubt is “I think, therefore I am.”