Cartesian dualism relies upon two substances, body and mind, which are totally
The answer to your question requires some deductive logic. Let's start with (1) all bodies are extended, which is Descartes' premise. It follows logically that if something is not extended, then it is not a body. Thus (3) follows logically from (1). You are worried because you think that (3) assumes that something is not extended. But it does not. It only claims that IF something is non extended then it is also not a body. So physicalists can agree with the claim.