Sextus Empiricus Pyrrhoniae Hypotyposes 1.14.124

[124] The Sixth Mode is that based on admixtures, by which we conclude that, because none of the real objects affects our senses by itself but always in conjunction with something else, though we may possibly be able to state the nature of the resultant mixture ¹ formed by the external object and that along with which it is perceived, we shall not be able to say what is the exact nature of the external reality in itself. That none of the external objects affects our senses by itself but always in conjunction with something else, and that, in consequence, it assumes a different appearance, is, I imagine, quite obvious.