From Book 10 of The Republic on the Immortality of the soul…
Socrates is talking to Glaucon:
Are you not aware, I [Socrates] said, that the soul of man is immortal and imperishable?
He [Glaucon] looked at me in astonishment, and said: No, by heaven: And are you really prepared to maintain this?
Yes, I said, I ought to be, and you too --there is no difficulty in proving it.
I see a great difficulty; but I should like to hear you state this argument of which you make so light….
Socrates: Would you agree with me in thinking that the corrupting and destroying element is the evil, and the saving and improving element the good?
Yes.
And you admit that every thing has a good and also an evil; as ophthalmia is the evil of the eyes and disease of the whole body; as mildew is of corn, and rot of timber, or rust of copper and iron: in everything, or in almost everything, there is an inherent evil and disease?
Yes, he said.
And anything which is infected by any of these evils is made evil, and at last wholly dissolves and dies?
True.
The vice and evil which is inherent in each is the destruction of each; and if this does not destroy them there is nothing else that will; for good certainly will not destroy them, nor again, that which is neither good nor evil.
Certainly not.
If, then, we find any nature which having this inherent corruption cannot be dissolved or destroyed, we may be certain that of such a nature there is no destruction?
That may be assumed.
Well, I said, and is there no evil which corrupts the soul?
Yes, he said, there are all the evils which we were just now passing in review: unrighteousness, intemperance, cowardice, ignorance.
But does any of these dissolve or destroy her? --and here do not let us fall into the error of supposing that the unjust and foolish man, when he is detected, perishes through his own injustice, which is an evil of the soul. Take the analogy of the body: The evil of the body is a disease which wastes and reduces and annihilates the body; and all the things of which we were just now speaking come to annihilation through their own corruption attaching to them and inhering in them and so destroying them. Is not this true?
Yes.
Consider the soul in like manner. Does the injustice or other evil which exists in the soul waste and consume her? Do they by attaching to the soul and inhering in her at last bring her to death, and so separate her from the body ?
Certainly not.
And yet, I said, it is unreasonable to suppose that anything can perish from without through affection of external evil which could not be destroyed from within by a corruption of its own?
It is, he replied.
Consider, I said, Glaucon, that even the badness of food, whether staleness, decomposition, or any other bad quality, when confined to the actual food, is not supposed to destroy the body; although, if the badness of food communicates corruption to the body, then we should say that the body has been destroyed by a corruption of itself, which is disease, brought on by this; but that the body, being one thing, can be destroyed by the badness of food, which is another, and which does not engender any natural infection --this we shall absolutely deny?
Very true.
And, on the same principle, unless some bodily evil can produce an evil of the soul, we must not suppose that the soul, which is one thing, can be dissolved by any merely external evil which belongs to another?
Yes, he said, there is reason in that.
Either then, let us refute this conclusion, or, while it remains unrefuted, let us never say that fever, or any other disease, or the knife put to the throat, or even the cutting up of the whole body into the minutest pieces, can destroy the soul, until she herself is proved to become more unholy or unrighteous in consequence of these things being done to the body; but that the soul, or anything else if not destroyed by an internal evil, can be destroyed by an external one, is not to. be affirmed by any man.
And surely, he replied, no one will ever prove that the souls of men become more unjust in consequence of death.
But if some one who would rather not admit the immortality of the soul boldly denies this, and says that the dying do really become more evil and unrighteous, then, if the speaker is right, I suppose that injustice, like disease, must be assumed to be fatal to the unjust, and that those who take this disorder die by the natural inherent power of destruction which evil has, and which kills them sooner or later, but in quite another way from that in which, at present, the wicked receive death at the hands of others as the penalty of their deeds?
Nay, he said, in that case injustice, if fatal to the unjust, will not be so very terrible to him, for he will be delivered from evil. But I rather suspect the opposite to be the truth, and that injustice which, if it have the power, will murder others, keeps the murderer alive --aye, and well awake too; so far removed is her dwelling-place from being a house of death.
True, I said; if the inherent natural vice or evil of the soul is unable to kill or destroy her, hardly will that which is appointed to be the destruction of some other body, destroy a soul or anything else except that of which it was appointed to be the destruction.
Yes, that can hardly be.
But the soul which cannot be destroyed by an evil, whether inherent or external, must exist for ever, and if existing for ever, must be immortal?
Certainly.
That is the conclusion, I said; and, if a true conclusion, then the souls must always be the same, for if none be destroyed they will not diminish in number. Neither will they increase, for the increase of the immortal natures must come from something mortal, and all things would thus end in immortality.
Very true.
But this we cannot believe --reason will not allow us --any more than we can believe the soul, in her truest nature, to be full of variety and difference and dissimilarity.
What do you mean? he said.
The soul, I said, being, as is now proven, immortal, must be the fairest of compositions and cannot be compounded of many elements?
Certainly not.
Her immortality is demonstrated by the previous argument, and there are many other proofs; but to see her as she really is, not as we now behold her, marred by communion with the body and other miseries, you must contemplate her with the eye of reason, in her original purity; and then her beauty will be revealed, and justice and injustice and all the things which we have described will be manifested more clearly. Thus far, we have spoken the truth concerning her as she appears at present, but we must remember also that we have seen her only in a condition which may be compared to that of the sea-god Glaucus, whose original image can hardly be discerned because his natural members are broken off and crushed and damaged by the waves in all sorts of ways, and incrustations have grown over them of seaweed and shells and stones, so that he is more like some monster than he is to his own natural form. And the soul which we behold is in a similar condition, disfigured by ten thousand ills. But not there, Glaucon, not there must we look.
Where then?
At her love of wisdom. Let us see whom she affects, and what society and converse she seeks in virtue of her near kindred with the immortal and eternal and divine; also how different she would become if wholly following this superior principle, and borne by a divine impulse out of the ocean in which she now is, and disengaged from the stones and shells and things of earth and rock which in wild variety spring up around her because she feeds upon earth, and is overgrown by the good things of this life as they are termed: then you would see her as she is, and know whether she has one shape only or many, or what her nature is. Of her affections and of the forms which she takes in this present life I think that we have now said enough.