Knowledge of external things must be by the mind, not by the senses.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“Knowledge by the senses is confused, and shared with animals; but now I have stripped the wax of its clothes, and mentally perceive it naked. From my sensibly seeing the wax, my own existence follows with certainty, but not that of the wax. Knowledge of external things must be by the mind, not by the senses. This leads to a consideration of different kinds of ideas. The commonest of errors, Descartes says, is to think that my ideas are like outside things. (The word “idea” includes sense-perceptions, as used by Descartes.)” source