"Η θεωρία των ιδεών."

[JamesTWils]

So, if you have an ancient name, like Πλατων, do you decline it like an Ancient Greek noun or like Modern Greek?

[troll1995]

Here You can use either, ancient greek-like declination is more sophisticated, but used.

[Marionsche4]

He also came to my mind straight ahead! Great to meat ppl with some education.

[Yoel.]

If it's referring to Plato, we call it the theory of Forms.

[JamesTWils]

It's definitely the modern version of the Ancient Greek word Plato uses for the Forms, but it apparently would translate our word "idea" as well. I agree with you, though. If I saw this out of context, I would definitely say the Theory of Forms.

[MiltiadisA1]

The ideas' theory
the theory of ideas

[mizinamo]

The possessive with apostrophe-s and the possessive with "of" are not (always) interchangeable.

In general, apostrophe-s (or in the plural, often just an apostrophe after plural s) is used for animate possessors such as humans or animals (the man's book, the dog's tail), while inanimate possessors usually use "of" (the colour of the door; not usually the door's colour). The more abstract the possessor, the less good apostrophe-s will sound.

Here, "ideas" is an abstract noun and "the ideas' theory" sounds very odd to me, as if the ideas have thoughts of their own.

[JacobPast177]

"The theory of ideas" - is that really considered to be a possessive?

[Jon345104]

Is this a Greek expression if so what does it mean