  If Berkeley’s Likeness Principle, the thesis that an idea can only be like another idea, is granted, representationalist materialism is in serious trouble. For how are material objects now to be characterized? If material objects are supposed to be extended, solid, or colored, Berkeley will counter that these sensory qualities pertain to ideas, to that which is immediately perceived, and that the materialist cannot assert that material objects are like ideas in these ways. Many passages in the Principles and Dialogues drive home this point, arguing that matter is, if not an incoherent notion, at best a completely empty one.