Many processes can be modeled mathematically. Hurricanes, gene dispersion, baseball statistics, insect wingbeats, galaxy formation: really, the list is endless. All of these processes can be said to "follow rules." Computers follow rules in this sense. But when we say about various natural processes that they "follow rules," it is important to keep in mind that the phrase is here used figuratively: there is no conscious rule-following going on, the way there is, say, after you have just taught me a card game and I try to play it correctly. We often use this figurative sense of "rule-following" when describing processes in our own bodies. The retina, for example, is an on-board computer of a sort that measures the amplitude of light coming in to the eye and "encodes" this "information" for transmission to the brain. "Encodes" and (importantly) "information" are also figurative terms in this context. The eyeball no more literally (intentionally) encodes things than, say, a tree encodes its age in its tree-rings.
Brain processes are "rule-following" only in this figurative sense. Stomachs digest food, but they don't eat lunch. Persons eat lunch. Brains compute (or, they do whatever it is that they do that is the equivalent of digestion in the stomach: our Cartesian error is in the way of our seeing what exactly that is). Persons think. I can't think without my brain any more than I can eat lunch without my stomach, but that doesn't mean that there's a little person in my brain thinking any more than it does that there's a little person in my stomach eating. And the processes going on in my brain are no better explained by saying that there's a person in there "interpreting" than are my digestive processes by positing a micro-gourmand. Inside my head there's lots of "rule-following" going on, but there is no rule-following. Actual rule-following is done by persons, out in the world. Thus the savant is "rule-following" (computing with his brain), but he is not rule-following (thinking with his "mind').
(Thanks to Kevin Vond for a lively exchange on this topic. See the comments below and go to Kevin's website for more.)