Cogito ergo sum.

"I think, therefore I am," said Descartes. But the ordinary man does not stand and wonder how he knows he exists. He assumes his own existence and claims to know physical things as they exist independent of his perception of them. A philosopher may look upon this innocent acceptance as an 'audacious piece of metaphysical theorizing' not at all based on what he calls 'sense data'. The philosopher cannot accept the knowledge of his existence …

…that philosophers have come to the conclusion that we know ourselves more certainly and infallibly tthat we know others. The fundamental postulate of Cogito ergo sum is the a posteriori beginning of Descartes' a priori philosophy. But the maxims of empiricism mean little or less tto the ordinary man, just as men who steer their lives by dreams do not pause to wonder at Plato's question "What is more real, the dreamer or the dream?"