Here, in the first place, as we have observed in the notes on that book, the road above between the fire and the fettered men, indicates that there is a certain ascent in the cave itself from a more abject to a more elevated life. By this ascent, therefore Plato signifies the contemplation of dianoetic objects in the mathematical disciplines. For as the shadows in the cave correspond to the shadows of visible objects, and visible objects are the immediate images of dianoetic forms, or those ideas which the soul essentially participates, it is evident that the objects from which these shadows are formed must correspond to such as are dianoetic. It is requisite, therefore, that the dianoetic power exercising itself in these, should draw forth the principles of these from their latent retreats, and should contemplate them not in images, but as subsisting in herself in impartible involution.
In the next place he says, "that the man who is to be led from the cave will more easily see what the heavens contain, and the heavens themselves, by looking in the night to the light of the stars, and the moon, than by day looking on the sun, and the light of the sun." By this he signifies the contemplation of intelligibles: for the stars and their light are imitations of intelligibles, so far as all of them partake of the form of the sun, in the same manner as intelligibles are characterized by the nature of the good.
After the contemplation of these, and after the eye is accustomed through these to the light, as it is requisite in the visible region to see the sun himself in the last place, in like manner, according to Plato, the idea of the good must be seen the last in the intelligible region. He, likewise divinely adds, that it is scarcely to be seen; for we can only be conjoined with it through the intelligible, in the vestibule of which it is beheld by the ascending soul.
In short, the cold, according to Plato, can only be restored while on earth to the divine likeness, which she abandoned by her descent, and be able after death to reascend to the intelligible world, by the exercise of the cathartic and theoretic virtues; the former purifying her from the defilements of a mortal nature, and the latter elevating her to the vision of true being: for thus, as Plato says in the Timaeus, "the soul becoming sane and entire, will arrive at the form of her pristine habit." The cathartic, however, must necessarily precede the theoretic virtues; since it is impossible to survey truth while subject to the perturbation and tumult of the passions. For the rational soul subsisting as a medium between intellect and the irrational nature, can then only without revulsion associate with the intellect prior to herself, when she becomes pure from copassivity with inferior natures. By the cathartic virtues, therefore, we become sane, in consequence of being liberated from the passions as diseases; but we become entire by the reassumption of intellect and science as of our proper parts; and this is effected by contemplative truth. Plato also clearly teaches us that our apostacy from better natures is only to be healed by a flight from hence, when he defines in his Theaetetus philosophy to be a flight from terrestrial evils: for he evinces by this that passions are connascent with mortals alone. He likewise says in the same dialogue, "that neither can evil be abolished, nor yet do they subsist with the gods, but that they necessarily revolve about this terrene abode, and a mortal nature." For those who are obnoxious to generation and corruption can also be affected in a manner contrary to nature, which is the beginning of evils. But in the same dialogue he subjoins the mode by which our flight from evil is to be accomplished. "It is necessary," says he "to fly from hence thither: but the flight is a similitude to divinity, as far as is possible to man; and this similitude consists in becoming just and holy in conjunction with intellectual prudence." For it is necessary that he who wishes to run from evils, should in the first place turn away from a mortal nature; since it is not possible for those who are mingled with it to avoid being filled with its attendant evils. As therefore, through our flight from divinity, and the defluction of those wings which elevate us on high, we fell into this mortal abode, and thus became connected with evils, so by abandoning passivity with a mortal nature, and by the germination of the virtues, as of certain wings, we return to the abode of pure and true good, and to the possession of divine felicity. For the essence of many subsisting as a medium between daemoniacal natures, who always have an intellectual knowledge of divinity, and those beings who are never adapted by nature to understand him, it ascends to the former and descends to the latter, through the possession and desertion of intellect. For it becomes familiar both with the divine and brutal likeness, through the amphibious condition of its nature.
When the soul therefore has recovered her pristine perfection in as great a degree as is possible, while she is an inhabitant of earth by the exercise of the cathartic and theoretic virtues, she returns after death, as he says in the Timaeus, to her kindred star, from which she fell, and enjoys a blessed life. Then, too, as he says in the Phaedrus, being winged, she governs the world in conjunction with the gods. And this indeed is the most beautiful end of her labors. This is what he calls in the Phaedo, a great contest and a mighty hope. This is the most perfect fruit of philosophy to familiarize and lead her back to things truly beautiful, to liberate her from this terrene abode as from a certain subterranean cavern of material life, elevate her to ethereal splendors, and place her in the islands of the blessed.
From this account of the human soul, that most important Platonic dogma necessarily follows, that our soul essentially contains all knowledge, and that whatever knowledge she acquires in the present life, is in reality nothing more than a recovery of what a he once possessed. This recovery is very properly called by Plato reminiscence, not as being attended with actual recollection in the present life, but as being an actual repossession of what the soul had lost through her oblivious union with the body. Alluding to this essential knowledge of the soul, which discipline evocates from its dormant retreats, Plato says in the Sophista, "that we know all things as in a dream, and are again ignorant of them, according to vigilant perception." Hence too, as Proclus well observes, it is evident that the soul does not collect her knowledge from sensibles, nor from things partial and divisible discover the whole and the one. For it is not proper to think that things which have in no respect a real subsistence, should be the leading causes of knowledge to the soul; and that things which oppose each other and are ambiguous, should precede science which has a sameness of subsistence; nor that things which are variously mutable, should be generative of reasons which are established in unity; nor that things indefinite should be the causes of definite intelligence. It is not fit, therefore, that the truth of things eternal should be received from the many, nor the discrimination of universals from sensibles, nor a judgment respecting what is good from irrational natures; but it is requisite that the soul entering within herself, should investigate herself the true and the good, and the eternal reasons of things.
We have said that discipline awakens the dormant knowledge of the soul; and Plato considered this as particularly effected by the mathematical discipline. Hence, he asserts of theoretic arithmetic that it imparts no small aid to our ascent to real being, and that it liberates us from the wandering and ignorance about a sensible nature. Geometry too is considered by him as most instrumental to the knowledge of the good, when it is not pursued for the sake of practical purposes, but as the means of ascent to an intelligible essence. Astronomy also is useful for the purpose of investigating the fabricator of all things, and contemplating as in most splendid images the ideal world, and its ineffable cause. And lastly music, when properly studied, is subservient to our ascent, viz. when from sensible we betake ourselves to the contemplation of ideal and divine harmony. Unless, however, we thus employ the mathematical discipline, the study of them is justly considered by Plato as imperfect and useless, and of no worth. For as the true end of man according to his philosophy is an assimilation to divinity, in the greatest perfection of which human nature is capable, whatever contributes to this is to be ardently pursued; but whatever has a different tendency, however necessary it may be to the wants and conveniences of the mere animal life, is comparatively little and vile. Hence it necessary to pass rapidly from things visible and audible, to those which are alone seen by the eye of intellect. For the mathematical sciences, when properly studied, move the inherent knowledge of the soul; awaken its intelligence; purify its dianoetic power; call forth its essential forms from their dormant retreats; remove that oblivion and ignorance which are congenial with our birth; and dissolve the bonds arising from our union with an irrational nature. It is therefore beautifully said by Plato in the 7th book of his Republic, "that the soul through these disciplines has an organ purified and enlightened, which is blinded and buried by studies of a different kind, an organ better worth saving than ten thousand eyes, since truth becomes visible through this alone."
Dialectic, however, or the vertex of the mathematical sciences, as it is called by Plato in his Republic, is that master discipline which particularly leads us up to an intelligible essence. Of this first of sciences, which is essentially different from vulgar logic, and is the same with what Aristotle calls the first philosophy and wisdom, I have largely spoken in the introduction and notes to the Parmenides. Suffice it therefore to observe in this place, that dialectic differs from mathematical science in this, that the latter flows from, and the former is void of hypothesis. That dialectic has a power of knowing universals; that it ascends to good and the supreme cause of all; and, that it considers good as the end of its elevation; but that the mathematical science, which previously fabricates for itself definite principles, from which it evinces things consequent to such principles, does not tend to the principle, but to the conclusion. Hence Plato does not expel mathematical knowledge from the number of the sciences, but asserts it to be the next in rank to that one science which is the summit of all; nor does he accuse it as ignorant of its own principles, but considers it as receiving these from the master science dialectic, and that possessing them without any demonstration, it demonstrates from these its consequent propositions.
Hence Socrates, in the Republic, speaking of the power of dialectic, says that it surrounds all disciplines like a defensive enclosure, and elevates those that use it to the good itself, and the first unities; that it purifies the eye of the soul; establishes itself in true beings, and, the one principle of all things, and ends at last in that which is no longer hypothetical. The power of dialectic, therefore, being thus great, and the ends of this path so mighty, it must by no means be confounded with arguments which are alone conversant with opinion: for the former is the guardian of sciences, and the passage to it is through these, but the latter is perfectly destitute of disciplinative science. To which we may add, that the method of reasoning which is founded in opinion, regards only that which is apparent; but the dialectic method endeavors to arrive at the one itself, always employing for this purpose steps of ascent, and at last beautifully ends in the nature of the good. Very different therefore is it from the merely logical method, which presides over the demonstrative phantasy, is of a secondary nature, and is alone pleased with contentious discussions. For the dialectic of Plato for the most part employs divisions and analyses as primary sciences, and as imitating the progression of beings from the one, and their conversion to it again. It likewise sometimes uses definitions and demonstrations, and prior to these the definitive method, and the divisive prior to this. On the contrary, the merely logical method, which is solely conversant with opinion, is deprived of the incontrovertible reasonings of demonstration.
The following is a specimen of the analytical method of Plato's dialectic. Of analysis there are three species. For one is an ascent from sensibles to the first intelligibles; a second is an ascent through things demonstrated and subdemonstrated, to undemonstrated and immediate propositions; and a third proceeds from hypothesis to unhypothetical principles. Of the first of these species, Plato has given a most admirable specimen in the speech of Diotima in the Banquet. For there he ascends from the beauty about bodies to the beauty in souls; from this to the beauty in right disciplines; from this again to the beauty in laws; from the beauty in laws to the ample sea of beauty (Greek: to polu pelagos tou kalou); and thus proceeding he at length arrives at the beautiful itself.