Some amusing quotes: "A closely related problem is that of distinguishing between physical systems such as digital computers, which appear to compute, and phys

Some amusing quotes: "A closely related problem is that of distinguishing between physical systems such as digital computers, which appear to compute, and physical systems such as rocks, which appear not to compute. Unlike computers, ordinary rocks are not sold in computer stores and are usually not considered computers. Why? What do computers have that rocks lack, such that computers compute and rocks don't? (If indeed they don't?) In other words, what does it take for a computation to be implemented in a concrete physical system? Different answers to these questions give rise to different accounts of concrete computation." ... "Consider a rock under the sun, early in the morning. During any time interval, the rock's temperature rises. The rock goes from temperature T to temperature T+1, to T+2, to T+3. Now consider a NOT gate that feeds its output back to itself. At first, suppose the NOT gate receives ‘0’ as an input; it then returns a ‘1’. After the ‘1’ is fed back to the NOT gate, the gate returns a ‘0’ again, and so on. The NOT gate goes back and forth between outputting a ‘0’ and outputting a ‘1’. Now map physical states T and T+2 onto ‘0’; then map T+1 and T+3 onto ‘1’." According to the simple mapping account, the rock implements a NOT gate undergoing the computation represented by ‘0101’. By contast, according to the counterfactual account, the rock's putative computational implementation is spurious, because the physical state transitions do not support counterfactuals. If the rock were put in state T, it may or may not transition into T+1 depending on whether it is morning or evening and other extraneous factors. Since the rock's physical state transitions that map onto the NOT gate's computational state transitions do not support counterfactuals, the rock does not implement the NOT gate according to the counterfactual account."