Plotinos: Complete Works, v. 4 In Chronological Order,...

But how do we know that which is absolutely formless (matter)? We make abstraction of all kinds of form, and what remains we call matter. We allow ourselves to be penetrated by a kind of shapelessness by the mere fact that we make abstraction of all shape in order to be able to represent matter (by a "bastard reasoning").185 Consequently, intelligence becomes altered, and ceases to be genuine intelligence when it dares in this way to look at what does not belong to its domain.186 It resembles the eye, which withdraws from light to see darkness, and which on that very account does not see. Thus, in not seeing, the eye sees darkness so far as it is naturally capable of seeing it. Thus intelligence which hides light within itself, and which, so to speak, issues from itself, by advancing towards things alien to its nature, without bringing along its own light, places itself in a state contrary to its being to cognize a nature contrary to its own.165 But enough of this.