 Timaeus introduces the receptacle as a “third kind” (triton genos, 48e4) alongside forms and their imitations. He apologizes for the obscurity of the concept, and attempts to explicate its role by means of a series of analogies: it is variously compared to a lump of gold (50a4–b5), a mother that together with a father produces offspring (50d2–4, 51a4–5), a plastic, impressionable stuff (50c2–6, e7–51a1), and an ointment that serves as a neutral base for various fragrances (50e5–8). These images suggest that it is devoid of any characteristics in its own right (except those formal characteristics necessary to its role, such as malleability). The receptacle is posited as the solution to a problem: none of the observable particulars persists as this or that (for example as fire or water) over time. We observe the very thing that is fire at one time becoming air, and subsequently becoming water, etc., “transmitting their becoming to one another in a cycle, so it seems” (49c6–7). Thus the thing that appears as fire here and now is not fire in its own right: its fieriness is only a temporary characterization of it. What, then, is that thing in its own right? In a difficult and controversial passage Timaeus proposes a  solution:[16]  In its own right it is (part of) a totally characterless subject that temporarily in its various parts gets characterized in various ways. This is the receptacle—an enduring substratum, neutral in itself but temporarily taking on the various characterizations through traces of the four elements in it. The observed particulars just are parts of that receptacle so characterized (51b4–6).