Travelling is almost like talking with those of other centuries.

The principal effect of the passions is that they incite and persuade the mind to will the events for which they prepared the body.

The only thing that I know, is that I know nothing

Except our own thoughts, there is nothing absolutely in our power.

The rainbow is such a remarkable phenomenon of nature, and its cause has been so meticulously sought after by inquiring minds throughout the ages, that I could not choose a more appropriate subject for demonstrating how, with the method I am using, we can arrive at knowledge not possessed at all by those whose writings are available to us.

I think; therefore I am.

Doubt is the origin of wisdom