It is important to note that the proof of our souls' existence in Hades rests upon their being 'born again from the dead1. 'The dead' are those who 'have died', and who have therefore already lived. Every incarnation is thus a reincarnation. The shift of tense from 'the dead' (70c8, d4) to 'those who have died' (70c5, c9) suggests this—see Loriaux, 122—6. The soul could not be 'born again', it is assumed, unless it persisted in Hades between incarnations. Cf.77d3— 4. Here, once again (see previous note), it is apparent that 'be born' must be taken as predicated of the soul, and not of the living things. For a living thing can be born, in the literal sense, only once.
70d7— e4. The argument is here extended to plants and animals as well as human beings. The scope of'all things subject to coming-to- be' (d9) is not very clear. It could be interpreted broadly to include all things subject to any kind of change. That it is not limited to the genesis of living things becomes apparent when Socrates starts talking (70el—6) about the coming-to-be of opposites. For some of the properties mentioned are not peculiar to organisms or even to physical objects.
The noun here translated 'coming-to-be' (genesis) corresponds to the verb discussed above (see on 70c4—8), and shares its ambiguity. 'Coming-to-be' is a key concept, and will be more fully explored later—see on 95e7—96a5 (p. 170). In the present argument the noun will be used several times from 71al3 onwards for a process of change between each of a pair of opposite states. It has there been translated 'process of coming-to-be' or simply 'process'.
The concept of an 'opposite', though critical for the argument both here and later (102b—106e), is never defined, but is illustrated with the pairs 'beautiful' and 'ugly', 'just' and 'unjust'. These pairs are contrary rather than contradictory opposites. They may not both be truthfully asserted of a given subject at the same time, but they may both be truthfully denied. With such pairs inferences cannot be made from the denial of one member to the affirmation of the other. Cf. Symposium 201e-202b, Sophist 257b. Plato notices at 90a that such properties may lie upon a range. The denial of a predicate at one end of the range does not license the affirmation of a predicate at the other.
How does this affect the argument? 'Living' and 'dead' are not extremes, lying at either end of a range. Nor can things be 'very living' or 'very dead'. None the less, 'living' and 'dead' are contrary opposites. They may both be truthfully denied of a subject. For their logical asymmetry, compare 'married' and 'divorced'. To argue that what is living must earlier have been dead, and therefore still earlier living, is like arguing that someone who is, at a given time, married, must at some earlier time have been divorced, and therefore still earlier married. Yet just as someone not yet married cannot be divorced, so something that is not yet living cannot be dead. See also next note.
70e4—71all. All the adjectives used here are in comparative form. Hackforth (64) thinks that such pairs as larger and smaller 'weaken the argument, since they are not genuine opposites'. Since opposites are never defined, it is not clear what would constitute 'genuine' ones. But the use of comparatives seems actually to strengthen the argument at this point. Socrates could hardly infer from a thing's coming to be weak that it must previously have been strong, for it might have been neither. But he can plausibly argue that if a thing comes to be weaker (than it was before), it must previously have been stronger (than it is now). This inference is valid, provided that both comparatives are filled out in the appropriate way. An inference from, e.g., 'Simmias comes to be weaker than Socrates' to 'Simmias was stronger than he now is' would be invalid. Note also that where the comparative after 'comes to be' is filled out by 'than it was before', the difference between 'x comes to be F-er' and 'x is F-er' becomes unimportant. For the latter implies that x has come to be F-er, and therefore that it was G-er (than it is now). It is, perhaps, assumed that a thing's coming to be large is equivalent to its coming to be either larger than something else or larger than itself at some previous time. For the question whether the distinction between simple and comparative forms is recognized in the Phaedo, see on 100e5-101b8, 102al0-d4.
The next stage of the argument will enable Socrates to dispense with comparatives altogether. But no argument relying entirely upon them could, in fact, support the claim that 'all things come to be in this way, opposite things from opposites' (71a9—10). For some opposites, such as 'odd' and 'even', have no comparative form. And no argument based solely upon comparatives could cater for 'living' and 'dead', since these do not admit of degree.
'That which is weaker' and 'that which is faster' (71a3—4) have been used here for the definite article with a neuter singular adjective in comparative form. Greek uses 'the F' to denote either the property F-ness, or a thing characterized by that property, or the class of things so characterized. The second use is relevant here. A particular thing that comes to be weaker (or faster) must previously have been stronger (or slower). This point is implicit in the use of 'things' (alO), and will be made explicit later (103a-b). See on 103a4-c9.
Socrates' 'law of opposites' may look like an inductive generalization based on the instances he gives. But it should be recognized that it owes any plausibility it has to the concepts used in its formulation. Bluck (18—19, 57) regards the Cyclical Argument as relying on 'mechanistic' principles, and suggests that Plato offers it only for the benefit of those who find mechanistic arguments impressive. But the present law is not genuinely 'mechanistic', nor is it derived from 'the study of physical phenomena'. It takes no empirical study to discover that what comes to be larger must have been smaller, or that what comes to be worse must have been better. Nor do these truths apply only to physical phenomena.
71al2—b5. A new phase of the argument begins here. Between each of a pair of opposites there is a process of coming-to-be. Every such process requires the postulation of a converse process. For the noun here translated 'process of coming-to-be' see on 70d7—e4 and 95e7—96a5 (p. 170).
This passage seems to introduce a further argument to show that the living are born from the dead. After concluding at 71e2 that our souls exist in Hades, Socrates will insist that there must be a process of coming to life again, and will continue (72a4—6): 'In that way too, then, we're agreed that living people are born from the dead.' This suggests that the appeal to reciprocal processes is meant as a second, independent argument for the conclusion.
In fact, however, the claim that there must be processes in both directions is vital for any effective use of the principle that opposites come from opposites. For the principle that whatever comes to be F must previously have been G requires that there actually be such a process as 'coming to be F\ If, for a given opposite F, there is no such process as 'coming to be F', the argument will break down. If, for example, there is no such process as 'coming to be unripe', it cannot be inferred from a thing's coming to be unripe that it was previously ripe. For this reason the appeal to reciprocal processes is not really a separate argument, but is essential for the working of the Cyclical Argument as a whole. It is not, indeed, clear whether Socrates recognizes it as such. But if he does, this might explain why it is introduced here, and interwoven with the reasoning that ends at 71e2.