When Plato talks about the Forms and the Form of the Good, how did he know that things such as Forms existed? What gave Plato the idea / inspiration to spend such time talking about the Forms?
Thanks!
Plato uses the forms to corroborate his theory on soul, amongst other theories, as I’m currently studying this aspect I can give you a few ideas:
Plato believed in cyclic-regeneration, that opposites are the result of their opposties ; hot comes from cold, a waking person comes form a sleeping one, a dead person comes from a living one, a living soul comes from a dead one - and vice-versa…so our souls existed before us and will exist after us.
Plato uses cyclic generation to explain why we have an innate knowledge of the Forms, we have a concept of Equality although we never physically witness it. e.g. we will never see a perfect sphere or two rulers that are exactly the same in length, but we do understand what Equality is…because it is one of the Forms and Plato claims are souls are aquainted with the Forms before they enter our bodies. Other forms being things like Truth, Justice and Beauty
So I’d say he was pretty interested in why we seem to already know thing we weren’t taught in this life…if the world exploded tomorrow and we just floated about space talking to eachother (I know, it’s only an exmple, ignore the fact that we’d explode to!) could we still talk about cats and dogs? Even after they no-longer exist?? The answer is yes because those creatures belong to a greater Form of some kind (I think i may be goin of track here)
But essentially Plato was really obsessed with proving the immortality of the soul, which the theory of Forms supports
Hope I’ve been of some help
Cleo xxx 
Thank you Cleo…“the unexamined life is not worth living” indeed!!