Cogito Ergo Sum

After doubting everything, Descartes found his bedrock: "I think, therefore I am" (cogito ergo sum). The very act of doubting proves something exists that is doing the doubting. Even if an evil demon controls all your perceptions, the demon cannot make you think without you existing to think.

This is the one proposition that self-destructs when denied. Try to doubt your own existence and you prove it. The more vigorously you doubt, the more certainly you demonstrate a doubter exists. Descartes had found his single unshakeable truth.