if only material exists and ideas don't, then it's retarded because 1. materialism is already an idea. 2. when we say something about material of a thing (like a house is just some bunch of boards), then even when we say boards (so it's a material here) we still don't say about what they are made of but rather highlighting the essence of the boards (so only essence exists, material doesn't, and we can't even say something about material because when we try then we say about its idea/essence but not material itself). It's because material is non-being/non-existence. It doesn't exist.