(I said I'd make an argument every week. This is not my own. I credit Plantinga and Searle)

A physical thing has size and mass. If the materialists are right and our thoughts are physical, they might be located in neuronal firings. From their viewpoint, thoughts and beliefs have neurophysiological properties. Thoughts just are these neuronal firings. But each event, if it is a belief or thought, also must have a sort of content. This event is a belief about something. A belief points beyond itself. My belief that I am wearing pajamas late in the day has a content. Its content is the proposition "I am wearing pajamas late in the day." According to Plantinga, its content is what enables the belief true or false. It is either true or false that I am wearing pajamas. 

Here's the problem. How can an assemblage of neuronal firings make proposition-carrying beliefs? It seems to me that no matter how we measure out neuronal firings or weigh or read them, we'll never have the faintest suggestion of a thought or belief. A physical thing is not about anything. It doesn't have propositional content. A physical thing is neither true nor false. It doesn't "rise to the dignity of an error."

Maybe we can think of a mind as a software program determined by the "hardware" of the brain. But the two are wholly distinct. "Since programs are defined purely formally or syntactically and since minds have an intrinsic mental content, it follows immediately that the program by itself cannot constitute the mind." (Searle, "Is the Brain a Digital Computer?") The symbols, rules and operations of a program (which represent the structure of a brain and the laws of physics that govern it) do not guarantee any semantic content. So a mind cannot be inferred from observing a brain. Since minds do exist, they cannot be one and the same thing as brains.