The Problems of Philosophy: Brief and Accessible Guide to the Problems of Philosophy - Bertrand Russell's Philosophical Exploration: Navigating the Problems of Philosophy

by Bertrand Russell

Is there any knowledge in the world which is so certain that no reasonable man could doubt it?

Whatever can be thought of is an idea in the mind of the person thinking of it; therefore nothing can be thought of except ideas in minds; therefore anything else is inconceivable, and what is inconceivable cannot exist.'