This will likely be my last post on the Affinity Argument of the Phaedo, which isn't stirring up a lot of interest, it seems.
I claimed yesterday that the Affinity Argument should not be understood, as usual, as an 'argument from analogy'. What is it then?
Let's say that an argument from analogy is one that notes similarities or resemblances between one thing or another, and then, on the basis of that, concludes that the two will be similar in other respects as well. Such an argument has all of the reliability of an induction based on accidental features ('This swan is white, and so it this, and so is this...')--that is, no reliability. Plato simply couldn't have been interested in an argument of that sort. He wasn't foolish enough to think than any serious form of reasoning could have that form.
What the Affinity Argument does aim to present us with--and in this respect it is characteristically Platonic--is an argument about the nature of things. Plato's claim is that the nature of the soul is such that we should reasonably group it with things the nature of which is not to perish.
He begins by dividing all that exists into two classes, the composite and the non-composite. Since he thinks that anything composite will be variable, he can then conclude from the fact that something is not variable, that it is non-composite. Thus, he works with the two classes: variable and invariable.
He thinks that the things in these different classes have different natures. (It doesn't just happen to be the case that something is in one class rather than another.) He then considers in which of these classes the soul is more reasonably placed, by considerations that have a bearing on the nature of the soul.
He gives three such considerations:
- The soul is naturally not 'visible', whereas changeable things are 'visible'.
- The soul is the sort of thing that, when it acts on its own and is free from constraints placed on it by the body, or by care of the body, then it tends to 'depart' (oichetai) to the Forms, until it actually 'makes contact' with them.
- The soul is naturally ordered to rule over the body.
The appeal to nature in the first consideration is contained in the line 79b6:
a)lla\ mh\n h(mei=j ge ta\ o(rata\ kai\ ta\ mh\ th=| tw=n a)nqrw/pwn fu/sei e)le/gomen: h)\ a)/llh| tini\ oi)/ei;"But we call things visible and invisible with reference to the nature human beings, do we not?"
Perhaps you've wondered about this before. Why the reference to specifically human vision? Plato's thought is that human sense organs are naturally adapted to perceive mutable, perishable things; whereas the human mind is naturally adapted (it is a 'natural instrument') for sensing unchanging, imperishable things. Each 'sense' has its natural objects. It follows that if something falls under the latter, but not the former, then it belongs in the class of unchanging, imperishable things.
Nature enters into the second consideration in just the way that H. Lorenz explains in his excellent SEP entry: we discern the nature of the soul when it is in circumstances such that it acts according to principles internal to it, and in such circumstances we see that it flees to be with the Forms. (It's as if: suppose whenever we let out an animal, it ran away to dwell in the forest; this would indicate that it was wild, that it did not belong at home.)
Nature enters fairly obviously into the third consideration, as it is Plato's claim (echoed repeatedly in the Republic and the elsewhere) that nature directs the soul to exercise authority over the body: tw=| me\n douleu/ein kai\ a)/rxesqai h( fu/sij prosta/ttei, th=| de\ a)/rxein kai\ despo/zein: kai\ kata\ tau=ta au)= po/tero/n soi dokei= o(/moion tw=| qei/w| ei)=nai kai\ po/teron tw=| qnhtw=|; (80a2).