“What is what must be.”

Our reasonings are grounded upon two great principles, that of contradiction, in virtue of which we judge false that which involves a contradiction, and true that which is opposed or contradictory to the false.

The larger the mass of collected things, the less will be their usefulness. Therefore, one should not only strive to assemble new goods from everywhere, but one must endeavor to put in the right order those that one already possesses.

Take what you need, do what you should, you will get what you want.

Of what use would it be to you, sir, to become King of China on condition that you forgot what you have been? Would it not be the same as if God, at the same time he destroyed you, created a King in China?

I have said more than once, that I hold space to be something purely relative, as time; an order of coexistences, as time is an order of successions.

A distinction must be made between true and false ideas, and that too much rein must not be given to a man’s imagination under pretext of its being a clear and distinct intellection.

He who understands Archimedes and Apollonius will admire less the achievements of the foremost men of later times.

Music is the pleasure the human mind experiences from counting without being aware that it is counting.

I hold that the mark of a genuine idea is that its possibility can be proved, either a priori by conceiving its cause or reason, or a posteriori when experience teaches us that it is in fact in nature.

We may say, that not only the soul (the mirror of an indestructible universe) is indestructible, but also the animal itself is, although its mechanism is frequently destroyed in parts.

There never is absolute birth nor complete death, in the strict sense, consisting in the separation of the soul from the body. What we call births are developments and growths, while what we call deaths are envelopments and diminutions.

Nothing is more important than to see the sources of invention which are, in my opinion more interesting than the inventions themselves.

Justice is charity in accordance with wisdom.

I also readily admit that there are animals, taken in the ordinary sense, that are incomparably larger than those we know of, and I have sometimes said in jest that there might be a system like ours which is the pocketwatch of some enormous giant.

To love is to be delighted by the happiness of someone, or to experience pleasure upon the happiness of another. I define this as true love.

Either there are no corporeal substances, and bodies are merely phenomena which are true or consistent with each other, such as a rainbow or a perfectly coherent dream, or there is in all corporeal substances something analogous to the soul.