For perception is a material and passive criterion, while reason (logos) is a formal one and the cause from which motion and being come; on that account sensation, in judging (krinein) by its affection and materially, apprehends things approximately and to the extent that the sense object makes its mark upon it, and signals no more than this, while reason (logos), in operating at a formal and immaterial level, is found to have [39] preconceived (proeilephos) the whole of what is being judged and virtually to possess accurately within itself the form (eidos) of what is investigated – more accurately than when it is observed among sensible objects. And so reason actually supplies what is missing from it [the form as supplied by the senses] and corrects what is not quite right; but it would not be able to do this if it did not have prior possession of it.[Porphyry On Ptolemy’s Harmonics 14,32-15,6 During]