Diotima of Mantinea plays an important role in Plato's Symposium. Since our only source concerning her is Plato, we cannot be certain whether she was a real historical personage or merely a fictional creation. However, it should be noted that nearly all of the characters named in Plato's dialogues have been found to correspond real people living during the time in question.
In Plato's Symposium, Socrates says that Diotima was a seer or priestess who had, in Plato's youth, taught him "the philosophy of love". Socrates also claims that she had succeeded in convincing the gods to postpone the pestilence that besieged Athens for ten years.
In Symposium Socrates relates a speech about love that he once heard from the priestess Diotima, who, Socrates claims, is the one who taught him all about love (201d). Love, in fact, is not a god, as he desires beautiful and good things, which the gods already have (202d). Neither is he mortal- just as love's lack of beauty does not make him ugly (202a), not being a god does not make him a mortal. Rather, he is in between mortal and immortal, like all spiritual things (202d–e); these spiritual things carry messages between the worlds of god and men, and "[b]eing in the middle of the two, the round out the whole and bind fast the all to that all" (202e). He is also a lover of wisdom, being in between wisdom and ignorance (204a).
Based on all that she had to explain to Socrates, Diotima concludes that Socrates was mistaking love for being loved rather than being a lover; this is why love struck him as beautiful in every way (204c). It seems that this would apply to at least some of the speeches preceding Socrates' as well – Phaedrus, for instance, clearly describes being loved rather than being a lover.
They go on to determine that "love is wanting to possess the good forever" (206b) and that the object of love is "giving birth in beauty, whether in body or soul" (206b). Furthermore, the lover desires not just to have beauty, but for "reproduction and birth in beauty" (206e); as a lover desires to possess the good forever, he seeks immortality as well, and achieving this is possible for mortals only in reproduction (207d).
She then outlines the difference between being pregnant in body and pregnant in soul. Those pregnant in body see childbirth as the way to provide themselves with "immortality and remembrance and happiness", while those pregnant in soul are pregnant with "what is fitting for a soul to bear and bring to birth . . . Wisdom and the rest of virtue, which all poets beget, as well as all the craftsmen who are said to be creative" (208e–209a).
The path taken by one pregnant in soul is then given; she then tells Socrates that "these rites [of love] when they are done correctly . . . is the final and highest mystery, and i don't know if you are capable of it" (210a). One pregnant in this way must devote himself to beautiful bodies in his youth; he will then see that ultimately "the beauty of all bodies is one and the same" (210a–b). With this, one comes to see that the beauty of one's soul is more valuable, and seeks to help young men pregnant in soul to bring birth to ideas (210b–c). The lover then sees the beauty of activities and laws, and then moves on to knowledge (210d–e).
It is here that Diotima explains the conclusion and purpose of these rites.
The result is that he will see the beauty of knowledge . . . the lover is turned to the great sea of beauty, and gazing upon this, he gives birth to many gloriously beautiful ideas and theories, in unstinting love of wisdom, until, having grown and been strengthened there, he catches sight of such knowledge, and it is the knowledge of such beauty . . .
. . . The man . . . who has beheld beautiful things in the right order and correctly, is now coming to the goal of Loving: all of a sudden he will catch sight of something wonderfully beautiful in its nature; that, Socrates, is the reason for all his earlier labors:
First, it always is and neither comes to be nor passes away, neither waxes nor wanes. Second, it is not beautiful this way and ugly that way, nor beautiful at one time and ugly at another, nor beautiful in relation to one thing and ugly in relation to another; nor is it beautifull here but ugly there, as it would be if it were beautiful for some people and ugly for others . . . It will not appear to him as one idea or on kind of knowledge. It is not anywhere in another thing, as in animal, or in earth, or in heaven, or in anything else, but itself by itself with itself . . . So when someone rises by these stages, through loving boys correctly, and begins to see this beauty, he has almost grasped his goal. This is what it is to go aright, or be led by another, into the mystery of Love: one goes always upward for the sake of this Beauty . . . and from these lessons he arrives in the end at this lesson, which is learning of this very Beauty, so that in the end he comes to know just what it is to be beautiful.
The purpose of all of this, she explains, is that only one who has seen Beauty in this manner can give birth to true virtue rather than images; the Love of the gods belongs to these people, and "if any human being could become immortal, it would be he" (212a).
Source: Wikpedia