At Timaeus 48e2–52d4 Timaeus sets out to establish that there are three principles or kinds (γένη) underlying the creation of the cosmos, not just the two he acknowledged earlier (27d–28a). The way he does so is not simply by adding an account of the third kind to the accounts of being and becoming that he has already given. Rather he does so by showing how each of the three differs from the others. It has not been noticed how this procedure structures the receptacle passage. The passage divides up into three parts, each punctuated by a list of the three kinds in a significant order. If we take X, Y and Z to stand for ‘becoming’, ‘being’ and ‘receptacle’ (or ‘space’, χώρα)’ respectively, the structure is (1) X≠Z, (2) X≠Y, (3) Y≠Z. By showing the distinctness of each pair, Timaeus demonstrates that all the kinds are distinct and that they are indeed three in number.

The first list occurs at 50c7–d2: the three kinds are listed in order as ‘what comes into being, that in which it comes into being, and that from which what comes into being is made in likeness.’ This list comes at the end of the passage which identifies the receptacle by differentiating it from what comes into being. The two first items on the list thus indicate the kinds that the passage has focussed on differentiating. The next list is given at 51e6–52b5, following on Timaeus' argument that being and becoming are different. Here the order is: ‘being, becoming, receptacle’, again with the items just distinguished in the first two places. Finally, at 52d2–4 Timaeus lists the three as ‘being, receptacle and becoming’, having just argued for the distinction between being and the receptacle. In the entire receptacle passage, the three kinds are accounted for, then, in three different ways.Footnote ¹

Without entering into the interpretation of the three kinds, let me try to say just enough to substantiate that each of the three sections of the passage serves to distinguish two of the three items.

(1) First is the argument that becoming differs from the receptacle. At 49a1–50c6 Timaeus introduces the receptacle by means of an aporia about the four ‘elements’. We see fire, earth, water and air continually changing into each other. In these circumstances, the safest thing, Timaeus tells us, is to say that none of them is this thing rather than that, say water rather than air. Rather we should take a lesson from the case of a lump of gold being continually reshaped and we should refer only to that in which the shapes appear as this thing (τοῦτο), while the changing appearances should only be referred to as the ‘such like’ (τὸ τοιοῦτον). This first section, then, articulates the role of the third kind as a receptacle by distinguishing it from the appearances in it.Footnote ²

(2) The proof that being ≠ becoming. At 51b6–52b5 Timaeus considers whether there exist a fire itself by itself and other similar forms over and above the sensible appearances. Some motivation for this question can be found in an unclarity in the previous passage about what it is that appears in the receptacle: the appearances occur when the forms imprint themselves on the receptacle, but, one may ask, does that mean that the forms somehow are the same as these appearances? No, argues Timaeus: given the difference between understanding (νοῦς) and true belief (δόξα ἀληθής), their objects, being and becoming, must be distinct.

(3) This leaves us with the difference between being and the receptacle in 52b6-d1. Granted, it is not obvious that this relationship is tackled in this passage, nor why Timaeus should want to address it: is it not clear, after all, that the forms must differ from the amorphous entity described at 50e–51a? However, the descriptions of the forms and the receptacle at the end of the previous section (52a1–b5) may suggest an overlap of properties between the two: the form is ungenerated (ἀγέννητον) and indestructible (ἀνώλεθρον), not entering into anything else, imperceptible and graspable by intellect (52a1–3), while the receptacle is eternal (ὂν τὸ τῆς χώρας ἀεί), not allowing destruction (φθορὰν οὐ προσδεχόμενον), and imperceptible, while graspable by a sort of (albeit corrupt) reasoning (μετ’ ἀναισθησίας ἁπτὸν λογισμῷ τινι νόθῳ) (52a8–b2). As F.M. Cornford observes, ‘[s]pace … is “everlastingly existent and not admitting destruction”. The phrase differs only verbally from that applied to the Form, “ungenerated and indestructible”.’Footnote ³ It is this surface similarity between form and receptacle which motivates Timaeus to conclude (52b6–d1) by stressing their difference and twoness. The thrust of this difficult text is that, while we think that it is a mark of being to be somewhere, to be in some place or space (52b4–5), this is wrong, for the appearances of the forms, which are somewhere, barely cling on to any being, while it is not true of the forms and the receptacle, which are, that either occurs in the other. For the true logos declares that ‘so long as the two things are different, neither can ever come to be in the other in such a way that the two should become at once one and the same thing and two’ (52b6–d1). As Cornford comments, ‘the language is obscure, but the whole drift of the passage demands that the two things in question must be the Form and Space. These must remain forever distinct.’Footnote ⁴

We have seen, then, that none of the kinds can be reduced to any of the others, and so (52d2–4) Timaeus casts his vote for their threeness.Footnote ⁵