Each problem that I solved became a rule, which served afterwards to solve other problems.
How do we know that anything really exists, that anything is really the way it seems ot us through our senses?
I should consider that I know nothing about physics if I were able to explain only how things might be, and were unable to demonstrate that they could not be otherwise.
The object of music is a Sound. The end; to delight, and move various Affections in us.
The nature of matter, or body considered in general, consists not in its being something which is hard or heavy or coloured, or which affects the senses in any way, but simply in its being something which is extended in length, breadth and depth.
Hence reason also demands that, since our thoughts cannot all be true because we are not wholly perfect, what truth they do possess must inevitably be found in the thoughts we have when awake, rather than in our dreams.
Even those who have the weakest souls could acquire absolute mastery over all their passions if we employed sufficient ingenuity in training and guiding them.
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?
All is to be doubted.
Desire awakens only to things that are thought possible.
Although my knowledge grows more and more, nevertheless I do not for that reason believe that it can ever be actually infinite, since it can never reach a point so high that it will be unable to attain any greater increase.
We call infinite that thing whose limits we have not perceived, and so by that word we do not signify what we understand about a thing, but rather what we do not understand.
But in my opinion, all things in nature occur mathematically.
A person has two passions for love and abhorrence. A big disposition to excessiveness has just a love, because it is more ardent and stronger.
It is possible that I am dreaming right now and that all of my perceptions are false.
Neither divine grace nor natural knowledge ever diminishes freedom.
If I found any new truths in the sciences, I can say that they follow from, or depend on, five or six principal problems which I succeeded in solving and which I regard as so many battles where the fortunes of war were on my side.
He lives well who is well hidden.
When writing about transcendental issues, be transcendentally clear.
And thereby make ourselves, as it were, the lords and masters of nature.
But I cannot forget that, at other times I have been deceived in sleep by similar illusions; and, attentively considering those cases, I perceive so clearly that there exist no certain marks by which the state of waking can ever be distinguished from sleep, that I feel greatly astonished; and in amazement I almost persuade myself that I am now dreaming.