"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 unless it is logically deduced.
Descartes undertook the task of doubting everything that could be legitimately be doubted. …
