我们本来是完整的，对于那种完整的希冀和追求就是所谓爱情。
Love is simply the name for the desire and pursuit of the whole.
柏拉图《会饮篇》：
从前人与现在人不一样：第一，从前人类有三种人，不像现在只有两种。在男人和女人之外，还有一种不男不女亦男亦女的人。这种人现在已经绝迹，但名称还保留着：‘阴阳人’，他们在形体上和名称上都是阴阳两性的。第二，从前人的形体是一个圆团，腰和背都是圆的，每人有四只手、四只脚，头和颈也是圆的，头上有两副面孔，前后方向相反，耳朵有两个，生殖器有一对，其他器官的数目都依比例加倍。为什么从前人有三种，身体如此构造呢？因为男人由太阳生出，女人由大地生出，阴阳人由月亮生出，因为月亮同时具备太阳和大地的性格。这种人的体力和精力都非常强壮，因此自高自大，乃至于图谋向诸神造反。于是宙斯和众神商量对付办法，他们不能灭绝人类，否则就没有人类对神的崇拜和牺牲祭祀。但人类的蛮横无礼不能容忍。宙斯想出一个办法，一方面让人类活着；另一方面削弱他们的力量，使他们不敢再捣乱。办法是把每个人截成两半，这样他们的力量削弱了，而数目加倍了，这样侍奉神的人和献给神的礼物也就加倍了。截开以后，他们只能用两只脚走路。把人截开后，宙斯叫阿波罗把人的面孔和半边颈项转到截开那一面，把截开的皮从两边拉到中间，拉到现在的肚皮地方，把缝口在肚皮中央系起，造成现在的肚脐。然后把皱纹弄平，只在肚皮和肚脐附近留下几条皱纹。
原来人这样被截成两半之后，这一半想念那一半，想再合拢在一起，饭也不吃，事也不做，直到饿死为止。若是这一半死了，那一半还活着，活着的那一半就到处寻求匹偶，一碰到就跳上前去拥抱，不管那是全女人截开的一半（就是我们现在所谓的女人），还是全男人截开的一半。这样，人类就消灭掉了。宙斯起了慈悲心，就想出一个新办法，把人的生殖器移到前面 － 从前都是在后面，生殖不是借男女交媾，而是把卵下到土里，像蝉一样 － 使男女可以借交媾来生殖。由于这种安排，如果抱着相合的是男人和女人，家会传下人种；如果抱着相合的是男人和男人，至少也可以平泄情欲，让心里轻松一下，好去从事人生的日常工作。就是像这样，从很古的时代，人与人彼此相爱的情欲就种植在人心里，它要恢复原始的整一状态，把两个人合成一个，医好从前截开的伤痛。 凡是由阴阳人截开的男人就成为女人的追求者，男情人大半是这样起来的，至于截开的女人也就成为女情人，男人的追求者。凡是由原始女人截开的女人对于男人就没有多大的兴趣，只眷恋和自己同性的女人，于是有女子同性爱者。凡由原男人切开的一半而成的男人，寻的都是男的；还是少男的时候，他们就亲近男人——因为他们是由原男人切开的一片，爱和男人做朋友，喜欢和男的一起睡，乃至于互相、拥抱交缠在一起。在少男和小伙子当中， 这种男人通常最优秀， 因为他们的自然最具男人气。这就是“娈童”和“象姑”们。他们在少年男子中大半是最优秀的，因为具有最强烈的男性。有人骂他们为无耻之徒，其实这是错误的，因为他们的行为并非由于无耻，而是由于强健勇敢，富于男性，急于追求同声同气的人。最好的证明是只有这批少年到了成年之后，才能在政治上显出是男子汉大丈夫。一旦到了壮年，他们所爱的也就是少年男子，对于娶妻生养子女没有自然的愿望，只是随着习俗去做；他们自己倒宁愿不结婚，常和爱人相守。总之，这种人的本性就是只爱同性男子，原因是要“同声相应，同气相求”。
如果这样一个人，无论它是少年男子的恋爱者，还是另一种恋爱者，碰巧遇到另一个人恰是他自己的另一半，那就会发生什么样的情形呢？他们就会马上互相爱慕，互相亲昵，一刻都不肯分离。他们终生在一起过共同的生活，可是彼此想从对方得到什么好处，却说不出。没有人会相信，只是由于共享爱情的乐趣，就可以使他们这样热烈地相亲相爱，很显然，两人心中都在愿望着一种隐约感觉到而说不出来的另一种东西。……他们每个人都会想，这正是他们许久以来所渴望的事，就是和爱人熔成一片，使两个人合成一个人。
(Plato, The Symposium)
In the first place, let me treat of the nature of man and what has happened to it; for the original human nature was not like the present, but different. The sexes were not two as they are now, but originally three in number; there was man, woman, and the union of the two, having a name corresponding to this double nature, which had once a real existence, but is now lost, and the word "Androgynous" is only preserved as a term of reproach.
In the second place, the primeval man was round, his back and sides forming a circle; and he had four hands and four feet, one head with two faces, looking opposite ways, set on a round neck and precisely alike; also four ears, two privy members, and the remainder to correspond. He could walk upright as men now do, backwards or forwards as he pleased, and he could also roll over and over at a great pace, turning on his four hands and four feet, eight in all, like tumblers going over and over with their legs in the air; this was when he wanted to run fast. Now the sexes were three, and such as I have described them; because the sun, moon, and earth are three;-and the man was originally the child of the sun, the woman of the earth, and the man-woman of the moon, which is made up of sun and earth, and they were all round and moved round and round: like their parents. Terrible was their might and strength, and the thoughts of their hearts were great, and they made an attack upon the gods; of them is told the tale of Otys and Ephialtes who, as Homer says, dared to scale heaven, and would have laid hands upon the gods. Doubt reigned in the celestial councils. Should they kill them and annihilate the race with thunderbolts, as they had done the giants, then there would be an end of the sacrifices and worship which men offered to them; but, on the other hand, the gods could not suffer their insolence to be unrestrained.
At last, after a good deal of reflection, Zeus discovered a way. He said: "Methinks I have a plan which will humble their pride and improve their manners; men shall continue to exist, but I will cut them in two and then they will be diminished in strength and increased in numbers; this will have the advantage of making them more profitable to us. They shall walk upright on two legs, and if they continue insolent and will not be quiet, I will split them again and they shall hop about on a single leg." He spoke and cut men in two, like a sorb-apple which is halved for pickling, or as you might divide an egg with a hair; and as he cut them one after another, he bade Apollo give the face and the half of the neck a turn in order that the man might contemplate the section of himself: he would thus learn a lesson of humility. Apollo was also bidden to heal their wounds and compose their forms. So he gave a turn to the face and pulled the skin from the sides all over that which in our language is called the belly, like the purses which draw in, and he made one mouth at the centre, which he fastened in a knot (the same which is called the navel); he also moulded the breast and took out most of the wrinkles, much as a shoemaker might smooth leather upon a last; he left a few, however, in the region of the belly and navel, as a memorial of the primeval state. After the division the two parts of man, each desiring his other half, came together, and throwing their arms about one another, entwined in mutual embraces, longing to grow into one, they were on the point of dying from hunger and self-neglect, because they did not like to do anything apart; and when one of the halves died and the other survived, the survivor sought another mate, man or woman as we call them, being the sections of entire men or women, and clung to that. They were being destroyed, when Zeus in pity of them invented a new plan: he turned the parts of generation round to the front, for this had not been always their position and they sowed the seed no longer as hitherto like grasshoppers in the ground, but in one another; and after the transposition the male generated in the female in order that by the mutual embraces of man and woman they might breed, and the race might continue; or if man came to man they might be satisfied, and rest, and go their ways to the business of life: so ancient is the desire of one another which is implanted in us, reuniting our original nature, making one of two, and healing the state of man.
Each of us when separated, having one side only, like a flat fish, is but the indenture of a man, and he is always looking for his other half. Men who are a section of that double nature which was once called Androgynous are lovers of women; adulterers are generally of this breed, and also adulterous women who lust after men: the women who are a section of the woman do not care for men, but have female attachments; the female companions are of this sort. But they who are a section of the male follow the male, and while they are young, being slices of the original man, they hang about men and embrace them, and they are themselves the best of boys and youths, because they have the most manly nature. Some indeed assert that they are shameless, but this is not true; for they do not act thus from any want of shame, but because they are valiant and manly, and have a manly countenance, and they embrace that which is like them. And these when they grow up become our statesmen, and these only, which is a great proof of the truth of what I am saving. When they reach manhood they are loves of youth, and are not naturally inclined to marry or beget children,-if at all, they do so only in obedience to the law; but they are satisfied if they may be allowed to live with one another unwedded; and such a nature is prone to love and ready to return love, always embracing that which is akin to him. And when one of them meets with his other half, the actual half of himself, whether he be a lover of youth or a lover of another sort, the pair are lost in an amazement of love and friendship and intimacy, and would not be out of the other's sight, as I may say, even for a moment: these are the people who pass their whole lives together; yet they could not explain what they desire of one another. For the intense yearning which each of them has towards the other does not appear to be the desire of lover's intercourse, but of something else which the soul of either evidently desires and cannot tell, and of which she has only a dark and doubtful presentiment. Suppose Hephaestus, with his instruments, to come to the pair who are lying side, by side and to say to them, "What do you people want of one another?" they would be unable to explain. And suppose further, that when he saw their perplexity he said: "Do you desire to be wholly one; always day and night to be in one another's company? for if this is what you desire, I am ready to melt you into one and let you grow together, so that being two you shall become one, and while you live a common life as if you were a single man, and after your death in the world below still be one departed soul instead of two-I ask whether this is what you lovingly desire, and whether you are satisfied to attain this?"-there is not a man of them who when he heard the proposal would deny or would not acknowledge that this meeting and melting into one another, this becoming one instead of two, was the very expression of his ancient need. And the reason is that human nature was originally one and we were a whole, and the desire and pursuit of the whole is called love. There was a time, I say, when we were one, but now because of the wickedness of mankind God has dispersed us, as the Arcadians were dispersed into villages by the Lacedaemonians. And if we are not obedient to the gods, there is a danger that we shall be split up again and go about in basso-relievo, like the profile figures having only half a nose which are sculptured on monuments, and that we shall be like tallies.