Have you ever consciously realized that everything you can be aware of is an IDEA. All you can think of, all you can perceive is made of IDEAS. Most of them are related to space, to other entities and their relations.
Our whole view of the world is made of IDEAS.
How can you be sure that the content of the idea you perceive is really there and that there is not just an entity representing this idea you perceive?
My mind, my ego, my consciousness is an spiritual entity dealing with IDEAS!