 A closely related problem is that of distinguishing between physical systems such as digital computers, which do compute (if anything does), and physical systems such as rocks, which do not (or so it seems). 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 but rocks don’t? (If indeed they don’t?) In other words, what kinds of concrete systems can compute at all? Different answers to these questions give rise to different accounts of concrete computation.