Once we have an idea of it it slips by, the factual knowledge about it eludes us from its deeper meaning, a meaning which is out of reach of human knowledge. Viewing the forms in this manner is helpful. Just as we could not very well conceive of justice as a form, we can only know its practice, we aproach the idea of love and all forms. Diotima shows us that knowledge is found in many ways. We gain access to some through our perceptions, our senses.
Some knowledge is reached through insight, the mind.
And, Diotima explains, some is found between, like love. Diotima, the Greek Priestess, was central to Socrates conception of love and of his idea of the forms.