Why do we say "I think, therefore I am (Cogito, Ergo Sum)"?

Meaning:

That I am able to wonder whether I exist, proves that I do exist.

Background:

Rene Descarte's philosophical statement, "I think, therefore I am" appears in Latin in his Principles of Philosophy (1644): "Cogito ergo sum".

He later repeated the thought, this time in French, in Discourse on Method (1647): "Je pense donc je suis."

Descarte (1596-1650) was not the first to express this thought but was, perhaps, the first to fit it into such a neat 'sound-bite'.

Much earlier, Plato wrote of "the knowledge of knowledge" and Aristotle explained the idea in some detail:

"But if life itself is good and pleasant .... and if one who sees is conscious that he sees, one who hears that he hears, one who walks that he walks and similarly for all the other human activities there is a faculty that is conscious of their exercise, so that whenever we perceive, we are conscious that we perceive, and whenever we think, we are conscious that we think, and to be conscious that we are perceiving or thinking is to be conscious that we exist."

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