This is a soft question and as such is rather opinion based. Nor am I entirely sure how to phrase it and will therefore edit as advised. Finally I apologise if there is any other thread here that subsumes this. What I am looking for is a list of key philosophical ideas (with key being defined as what a beginner would necessarily have to understand to read works in that field) similar to a list of these proofs.
Answers should have only one key idea along with a brief explanation that attempts to capture the gist of the idea if possible (if not then links to a suitable website would be appreciated).
Context: My philosophy paper requires us to discuss the meaning of philosophy with reference to 'key philosophical ideas' and how they demonstrate this meaning.