I can doubt everything, except one thing, and that is the very fact that I doubt. Simply put - I think, therefore I am

How can you be certain that your whole life is not a dream?

Just as we believe by faith that the greatest happiness of the next life consists simply in the contemplation of this divine majesty, likewise we experience that we derive the greatest joy of which we are capable in this life from the same contemplation, even though it is much less perfect.

When I consider this carefully, I find not a single property which with certainty separates the waking state from the dream. How can you be certain that your whole life is not a dream?

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

So far, I have been a spectator in this theater which is the world, but I am now about to mount the stage, and I come forward masked.

Even the mind depends so much on temperament and the disposition of one's bodily organs that, if it is possible to find a way to make people generally more wise and more skilful than they have been in the past, I believe that we should look for it in medicine. It is true that medicine as it is currently practiced contains little of much use.