Are ideas real?

If I have the idea of an "apple" or its taste, - is the idea as real as the apple itself? Specifically - do the objects of the 6th consciousness depend on anything except the 6th consciousness? In normal philosophical terms - does the idea depend on the mind having the idea, in order to exist? I think these are all equivalent questions, because causation is the conventional truth - ideas, the ultimate - no causation, and the middle - the buddha nature or mind-body.