Plato is the author of several books: The Republic, The Banquet, In defense of Socrates. In his book The Banquet, is narrated the nature of love personified by the God Eros. It is debatable if Plato used the character of  Socrates to expose his own ideas, but  anyway Socrates was a historical character and teacher of Plato.
In the book, the guests of the banquet present their own speeches until they reach the climax of the book in the dialogue between Socrates and the priestess Diotimia. The priestess introduces Plato into the secrets of love, and to be able to impart this knowledge  she makes Socrates understand the importance of the intermediate states in the genres, that is, if something is not nice it is not necessary that it be ugly, and if it is not good it does not have to be bad.
Aristotle in his book Metaphysics expose more clearly the problema of the intermediate states. It is important to know when there is an intermediate state and when not.
Aristotle tells us that when two opposites belong to the same gender exists an intermediate state.
In the genre of the knowledge, ignorance and wisdom are opposites but they belong to the same genre so there is an intermediate state known as opinion. Diotimia exposes to Plato in the book The Banquet that opinion is not knowledge because it does not give reason or cause, but neither the opinion is ignorance because in something hits on reality.
Between good and evil happens something similar, they belong to the genre of the elections, so exists an intermediate state between good and evil dominated by passions, this is an intermediate state not mixed state. The existence of this state is fundamental to understand the mysteries of love.
What is the difference between a mixed state and an intermediate state? In a mixed state the properties are a blend of the properties of the opposites states. But this not happens in a intermediate state. The intermediate state have his uniques properties.
For example: liquid water is an intermediate state between ice and steam, but liquid water as intermediate state possesses the unique properties of the liquid, different from the gas and the solid.
Between good and evil there is an intermediate state characterized and determined by the predominance of the passions. The existence of this state is very important for the sciences of psychology and sociology.
Another issue is when two opposites belong to different genres. For example between truth and falsehood there is no intermediate state. Something is false or true.
In the book The Banquet of Plato, Diotimia ends his speech with the conclusion of the existence of the Love for the good called in Greek Agape, this term will be later taken up by Christianity to refer to the state of grace and the "holy spirit".
Summarizing the theory expounded by Plato in the Banquet, the God Eros would personify according to the guests' speeches three different forms of love, including an intermediate state between good and evil: the passion love vulgarly called Eros; Philia the love of the Friends or to  say the love of those who are similar, and finally the Agape or Good Love also called "Platonic Love". This is the idea of love that according to Plato taught to him Socrates.
The book exposes the "opinions about love and happiness" to end with a teaching, with a knowledge about the mysteries of love imparted by Socrates himself, the Wise, in his dialogue with Diotimia.