All is to be doubted.

Mathematics is a more powerful instrument of knowledge than any other that has been bequeathed to us by human agency.

The first precept was never to accept a thing as true until I knew it as such without a single doubt.

It is to the body alone that we should attribute everything that can be observed in us to oppose our reason.

Be that as it may, there is fixed in my mind a certain opinion of long standing, namely that there exists a God who is able to do anything and by whom I, such as I am, have been created. How do I know that he did not bring it about that there is no earth at all, no heavens, no extended thing, no shape, no size, no place, and yet bringing it about that all these things appear to me to exist precisely as they do now?

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.

Give me extension and motion and I will construct the universe.