How do we know that anything really exists, that anything is really the way it seems ot us through our senses?

Science is practical philosophy.

De omnibus dubitandum

Common sense is the most widely shared commodity in the world, for every man is convinced that he is well supplied with it.

Divide each difficulty at hand into as many pieces as possible and as could be required to better solve them.

Now therefore, that my mind is free from all cares, and that I have obtained for myself assured leisure in peaceful solitude, I shall apply myself seriously and freely to the general destruction of all my former opinions.

Even if I were to suppose that I was dreaming and whatever I saw or imagined was false, yet I could not deny that ideas were truly in my mind.