"Monads have no windows through which anything could enter or depart."

"The mind is not only capable of knowing [innate ideas], but further of finding them in itself; and if it had only the simple capacity to receive knowledge…it would not be the source of necessary truth…"

"Why is there anything at all rather than nothing whatsoever?"

"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 actual in nature."

"I do not like X as a symbol for multiplication, as it is easily confounded with x."