I have never really thought about it but now that I am, I disagree with that statement. Is it safe to assume that because you are thinking therefore you exist and those that don't think therefore are nonexistent?.Biology has thought us that a living organism that breathes, eat, uses energy for it's metabolism and reproduces is alive. Therefore plants, bacteria, viruses they all are alive, and clearly exist, but do they think as us human do????Therefore that statement is flawed.