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Rene Descartes – Key Philosophical Writings

 but in order that we may here distinguish that which is clear from that which is obscure we ought to observe that we have a clear or distinct knowledge of pain, colour and other things of the sort when we consider them simply as sensations or thoughts. 

But when we desire to judge of such matters as existing outside of our mind, we can in no wise conceive what sort of things they are.  And when any one says that he sees colour in a body or feels pain in one of his limbs, it is the same as if he told us that he there saw or felt something but was absolutely ignorant of its nature, or else that he did not know what he saw or felt. 

For although when he examines his thoughts with less attention he perhaps easily persuades himself that he has some knowledge of it, because he supposes that there is something resembling the sensation of colour or pain which he experiences, yet if he investigates what is represented to him by this sensation of colour or pain appearing as they do to exist in a coloured body or suffering part, he will find that he is really ignorant of it.