Bertrand Arthur William Russell (18 May 1872–2 February 1970)

"Pure mathematics consists entirely of assertions to the effect that, if such and such a proposition is true of anything, then such and such another proposition is true of that thing. It is essential not to discuss whether the first proposition is really true, and not to mention what the anything is, of which it is supposed to be true ... If our hypothesis is about anything, and not about some one or more particular things, then our deductions constitute mathematics. Thus mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true. People who have been puzzled by the beginnings of mathematics will, I hope, find comfort in this definition, and will probably agree that it is accurate."

Immanuel Kant (22 April 1724 – 12 February 1804)

"How is it possible to conceive an Extension of Pure Reason in a Practical point of view, with its knowledge as speculative being enlarge at the same time?"

David Hume (26 April 1711- 25 August 1776)

"It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us those powers and principles on which the influence of those objects entirely depends. "