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?
Rene Descartes
Certain
I suppose therefore that
I suppose therefore that all things I see are illusions; I believe that nothing has ever existed of everything my lying memory tells me. I think I have no senses. I believe that body, shape, extension, motion, location are functions. What is there then that can be taken as true? Perhaps only this one thing, that nothing at all is certain.
Rene Descartes
Intuition is the undoubting
Intuition is the undoubting conception of a pure and attentive mind, which arises from the light of reason alone, and is more certain than deduction.
Rene Descartes
…it is certain that
…it is certain that I am really distinct from my body, and can exist without it.
Rene Descartes
I was convinced that
I was convinced that our beliefs are based much more on custom and example than on any certain knowledge.
Rene Descartes
How can you be
How can you be certain that your whole life is not a dream?
Rene Descartes
For those [observations] that
For those [observations] that I made in Leipzig in my youth and up to my 21st year, I usually call childish and of doubtful value. Those that I took later until my 28th year [i.e., until 1574] I call juvenile and fairly serviceable. The third group, however, which I made at Uraniborg during approximately the last 21 years with the greatest care and with very accurate instruments at a more mature age, until I was fifty years of age, those I call the observations of my manhood, completely valid and absolutely certain, and this is my opinion of them.
Tycho Brahe
Behold, directly overhead, a
Behold, directly overhead, a certain strange star was suddenly seen . . . Amazed, and as if astonished and stupified, I stood still
Tycho Brahe