http://classics.mit.edu/Plato/meno.html
The soul being immortal is what I want to demonstrate. The line about the soul being reborn many times is false, because the soul is immortal, it can't die, so it can't be born either. The part the says "having seen all things that exist, whether in this world or in the world below, has knowledge of them all" is untrue, but it's close to being true.Soc. Then begin again, and answer me, What, according to you and your friend Gorgias, is the definition of virtue?
Men. O Socrates, I used to be told, before I knew you, that you were always doubting yourself and making others doubt; and now you are casting your spells over me, and I am simply getting bewitched and enchanted, and am at my wits' end. And if I may venture to make a jest upon you, you seem to me both in your appearance and in your power over others to be very like the flat torpedo fish, who torpifies those who come near him and touch him, as you have now torpified me, I think. For my soul and my tongue are really torpid, and I do not know how to answer you; and though I have been delivered of an infinite variety of speeches about virtue before now, and to many persons-and very good ones they were, as I thought-at this moment I cannot even say what virtue is. And I think that. you are very wise in not voyaging and going away from home, for if you did in other places as do in Athens, you would be cast into prison as a magician.
Soc. You are a rogue, Meno, and had all but caught me.
Men. What do you mean, Socrates?
Soc. I can tell why you made a simile about me.
Men. Why?
Soc. In order that I might make another simile about you. For I know that all pretty young gentlemen like to have pretty similes made about them-as well they may-but I shall not return the compliment. As to my being a torpedo, if the torpedo is torpid as well as the cause of torpidity in others, then indeed I am a torpedo, but not otherwise; for I perplex others, not because I am clear, but because I am utterly perplexed myself. And now I know not what virtue is, and you seem to be in the same case, although you did once perhaps know before you touched me. However, I have no objection to join with you in the enquiry.
Men. And how will you enquire, Socrates, into that which you do not know? What will you put forth as the subject of enquiry? And if you find what you want, how will you ever know that this is the thing which you did not know?
Soc. I know, Meno, what you mean; but just see what a tiresome dispute you are introducing. You argue that man cannot enquire either about that which he knows, or about that which he does not know; for if he knows, he has no need to enquire; and if not, he cannot; for he does not know the, very subject about which he is to enquire.
Men. Well, Socrates, and is not the argument sound?
Soc. I think not.
Men. Why not?
Soc. I will tell you why: I have heard from certain wise men and women who spoke of things divine that-
Men. What did they say?
Soc. They spoke of a glorious truth, as I conceive.
Men. What was it? and who were they?
Soc. Some of them were priests and priestesses, who had studied how they might be able to give a reason of their profession: there, have been poets also, who spoke of these things by inspiration, like Pindar, and many others who were inspired. And they say-mark, now, and see whether their words are true-they say that the soul of man is immortal, and at one time has an end, which is termed dying, and at another time is born again, but is never destroyed. And the moral is, that a man ought to live always in perfect holiness. "For in the ninth year Persephone sends the souls of those from whom she has received the penalty of ancient crime back again from beneath into the light of the sun above, and these are they who become noble kings and mighty men and great in wisdom and are called saintly heroes in after ages." The soul, then, as being immortal, and having been born again many times, rand having seen all things that exist, whether in this world or in the world below, has knowledge of them all; and it is no wonder that she should be able to call to remembrance all that she ever knew about virtue, and about everything; for as all nature is akin, and the soul has learned all things; there is no difficulty in her eliciting or as men say learning, out of a single recollection -all the rest, if a man is strenuous and does not faint; for all enquiry and all learning is but recollection. And therefore we ought not to listen to this sophistical argument about the impossibility of enquiry: for it will make us idle; and is sweet only to the sluggard; but the other saying will make us active and inquisitive. In that confiding, I will gladly enquire with you into the nature of virtue.
Men. Yes, Socrates; but what do you mean by saying that we do not learn, and that what we call learning is only a process of recollection? Can you teach me how this is?
Soc. I told you, Meno, just now that you were a rogue, and now you ask whether I can teach you, when I am saying that there is no teaching, but only recollection; and thus you imagine that you will involve me in a contradiction.
Socrates and Meno are discussing virtue, and Socrates is trying to get Meno to define virtue. Meno replied with a list of virtues, which doesn't answer the question of what virtue actually is. The same type of problem exists in a roundabout way in the statement: "having seen all things that exist, whether in this world or in the world below, has knowledge of them all". We do have knowledge of all things that exist because we have knowledge of existence itself. If we didn't, nothing would exist, at least for us, so it's necessarily the case that we have this knowledge.
This knowledge of existence constitutes our "immortal soul". Birth is an existence and death is an existence, so existence must be a lower abstraction that birth and death are built on, otherwise birth and death wouldn't exist. Looking at it another way, existence can't be subject to birth and death because that would mean after the death of existence, something could be born yet the birth wouldn't exist, which doesn't make sense. So knowledge of existence transcends knowledge of birth and death.
Now, onto the idea of recollection. I want to demonstrate that this is a valid idea, without the idea of a personal soul being reborn and experiencing different things in order to recollect. The recollection occurs with respect to the knowledge of existence, which we forget.
So, what is existence? That's the key upon which this demonstration rests. What is the fundamental essence that runs through all the particulars of existence? It's the law of identity, A=A. So later in the dialog, when Socrates is teaching the boy about the square of a square, he really is recollecting, but it's not about something he learned in a past life (unless you want to take "past life" in a poetical sense). He's recollecting about A=A. Specifically, he's recollecting that the area of a rectangle ('A') is ('=') the length times the width ('A') and there is no other way to do it. If you calculate it another way, then you're not calculating the area.
This is another way to view the immortality of the soul, although it's a bit more abstract, but the concept of "area" has a unique identity that can't be changed in any way. If you try to change it, like the boy unwittingly tried to do, then you end up with something that is not the area, but something else with a different identity. So "area" has a timeless nature about it, and our knowledge of timeless things like this is what constitutes the immortal part of our consciousness (soul). It goes back to our knowledge of existence. This knowledge is what everything we experience is built on at the most fundamental level and spirituality is a matter of recollecting this at all times.