The mind contemplates some things through its own instrumentality, others through the bodily faculties. … It follows that we cannot know things through the senses alone, since through the senses alone we cannot know that things exist. Therefore knowledge consists in reflection, not in impressions, and perception is not knowledge, because it ‘has no part in apprehending truth, since it has none in apprehending existence’.

Bertrand Russell, History of Western Philosop…, loc. 693