Ultimate appeal to hypothesis of extreme generality.
But if your opponent impugns the hypothesis itself, how are you to defend it? Plato here tells us: by means of some other hypothesis or assumption, yet more universal than itself. You must ascend upwards in the scale of generality, until you find an assumption suitable and sufficient.[80]
[80] Plato, Phædon, p. 101 E.
We here see where it was that Plato looked for full, indisputable, self-recommending and self-assuring, certainty and truth. Among the most universal propositions. He states the matter here as if we were to provide defence for an hypothesis less universal by ascending to another hypothesis more universal. This is illustrated by what he says in the Timæus — Propositions are cognate with the matter which they affirm: those whose affirmation is purely intellectual, comprising only matter of the intelligible world, or of genuine Essence, are solid and inexpugnable: those which take in more or less of the sensible world, which is a mere copy of the intelligible exemplar, become less and less trustworthy — mere probabilities. Here we have the Platonic worship of the most universal propositions, as the only primary and evident truths.[81] But in the sixth and seventh books of the Republic, he delivers a precept somewhat different, requiring the philosopher not to rest in any hypothesis as an ultimatum, but to consider them all as stepping-stones for enabling him to ascend into a higher region, above all hypothesis — to the first principle of every thing: and he considers geometrical reasoning as defective because it takes its departure from hypothesis or assumptions of which no account is rendered.[82] In the Republic he thus contemplates an intuition by the mind of some primary, clear, self-evident truth, above all hypotheses or assumptions even the most universal, and transmitting its own certainty to every thing which could be logically deduced from it: while in the Phædon, he does not recognise any thing higher or more certain than the most universal hypothesis — and he even presents the theory of self-existent Ideas as nothing more than an hypothesis, though a very satisfactory one. In the Republic, Plato has come to imagine the Idea of Good as distinguished from and illuminating all the other Ideas: in the Timæus, it seems personified in the Demiurgus; in the Phædon, that Idea of Good appears to be represented by the Nous or Reason of Anaxagoras. But Sokrates is unable to follow it out, so that it becomes included, without any pre-eminence, among the Ideas generally: all of them transcendental, co-ordinate, and primary sources of truth to the intelligent mind — yet each of them exercising a causative influence in its own department, and bestowing its own special character on various particulars.
[81] Plato, Timæus, p. 29 B. ὧδε οὖν περί τε εἰκόνος καὶ τοῦ παραδείγματος διοριστέον, ὡς ἄρα τοὺς λόγους, ὧνπέρ εἰσιν ἐξηγηταί, τούτων αὐτῶν καὶ ξυγγενεῖς ὄντας. τοῦ μὲν οὖν μονίμου καὶ βεβαίου καὶ μετὰ νοῦ καταφανοῦς, μονίμους καὶ ἀμεταπτώτους … τοὺς δὲ τοῦ πρὸς μὲν ἐκεῖνο ἀπεικασθέντος, ὄντος δὲ εἰκόνος, εἰκότας ἀνὰ λόγον τε ἐκείνων ὄντας· ὅ, τιπερ πρὸς γένεσιν οὐσία, τοῦτο πρὸς πίστιν ἀληθεία.
[82] Plato, Republic, vi. p. 511. τῶν ὑποθέσεων ἀνωτέρω ἐκβαίνειν .… τὸ ἕτερον τμῆμα τοῦ νοητοῦ, οὖ αὐτὸς ὁ λόγος ἄπτεται τῇ τοῦ διαλέγεσθαι δυνάμει, τὰς ὑποθέσεις ποιούμενος οὐκ ἀρχὰς ἀλλὰ τῷ ὄντι ὑποθέσεις, οἷον ἐπιβάσεις τε καὶ ὁρμάς, ἵνα μέχρι τοῦ ἀνυποθέτου ἐπὶ τὴν τοῦ παντὸς ἀρχὴν ἰών, ἁψάμενος αὐτῆς, πάλιν αὖ ἐχόμενος τῶν ἐκείνης ἐχομένων, οὕτως ἐπὶ τελευτὴν καταβαίνῃ, αἰσθητῷ παντάπασιν οὐδενὶ προσχρώμενος, ἀλλ’ εἴδεσιν αὐτοῖς δι’ αὐτῶν εἰς αὐτά, καὶ τελευτᾷ εἰς εἴδη. Compare vii. p. 533.
Plato’s demonstration of the immortality of the soul rests upon the assumption of the Platonic ideas. Reasoning to prove this.
It is from the assumption of these Ideas as eternal Essences, that Plato undertakes to demonstrate the immortality of the soul. One Idea or Form will not admit, but peremptorily excludes, the approach of that other Form which is opposite to it. Greatness will not receive the form of littleness: nor will the greatness which is in any particular subject receive the form of littleness. If the form of littleness be brought to bear, greatness will not stay to receive it, but will either retire or be destroyed. The same is true likewise respecting that which essentially has the form: thus fire has essentially the form of heat, and snow has essentially the form of cold. Accordingly fire, as it will not receive the form of cold, so neither will it receive snow: and snow, as it will not receive the form of heat, so neither will it receive fire. If fire comes, snow will either retire or will be destroyed. The Triad has always the Form of Oddness, and will never receive that of Evenness: the Dyad has always the Form of Evenness, and will never receive that of Oddness — upon the approach of this latter it will either disappear or will be destroyed: moreover the Dyad, while refusing to receive the Form of Oddness, will refuse also to receive that of the Triad, which always embodies that Form — although three is not in direct contrariety with two. If then we are asked, What is that, the presence of which makes a body hot? we need not confine ourselves to the answer — It is the Form of Heat — which, though correct, gives no new information: but we may farther say — It is Fire, which involves the Form of Heat. If we are asked, What is that, the presence of which makes a number odd, we shall not say — It is Oddness: but we shall say — It is the Triad or the Pentad — both of which involve Oddness.
The soul always brings life, and is essentially living. It cannot receive death: in other words, it is immortal.
In like manner, the question being asked, What is that, which, being in the body, will give it life? we must answer — It is the soul. The soul, when it lays hold of any body, always arrives bringing with it life. Now death is the contrary of life. Accordingly the soul, which always brings with it life, will never receive the contrary of life. In other words, it is deathless or immortal.[83]