Plotinos: Complete Works, v. 1 In Chronological Order,...

We must acknowledge that the reasons which produce these things (as heat, and whiteness) are beings, if taken in their totality; but on considering their production, we see that what constitutes a whatness or quiddity (the Aristotelian "what it were to be") in the intelligible world, becomes a quality in the sense-world. Consequently, we always err on the subject of the quiddity, when we try to determine it, mistaking the simple quality for it (as thought Plato),332 for, when we perceive a quality, the fire is not what we call fire, but a being. As to the things which arrest our gaze, we should distinguish them from the quiddity, and define them by the qualities of sense (objects); for they do not constitute the being, but the affections of being.