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

Illusory joy is often worth more than genuine sorrow.

Situations in life often permit no delay; and when we cannot determine the course which is certainly best, we must follow the one which is probably the best. This frame of mind freed me also from the repentance and remorse commonly felt by those vacillating individuals who are always seeking as worthwhile things which they later judge to be bad.

All is to be doubted.

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.

Archimedes, that he might transport the entire globe ... demanded only a point that was firm and immovable; so also, I shall be entitled to entertain the highest expectations, if I am fortunate enough to discover only one thing that is certain and indubitable.

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.