Commenting before reading other replies---I'm going to give the boring, sneaky reply that the question isn't well-specified enough to have an answer; I'd need to know more about what you mean by something to operate solely on a substrate. I mean, clearly there are a lot of cognitive tasks that most people can only do given a pencil and paper, or a computer ... is that the sneaky part, that we store information in the environment, and therefore we're not solely neurons?