In response to a previous thread,
Jonathan Livengood asked some very good questions about, roughly, what should count as information processing and computation in physical systems. Perhaps it will help to take a step back.
In my early work on computation, I argued that, roughly,
only physical processes that take strings of digits as inputs and return strings of digits as outputs by following a rule defined over the inputs (and possibly internal states) count as computing systems . My reason had to do with the centrality of the formalisms of computability theory to the notion of computation. As to analog computers, which do not manipulate strings of digits but are still called computers, I argued that
they are "computational" only by courtesy and for contingent historical reasons .
I later realized that, although there was something right about my early purism about computation, it was unhelpful to try to restrict the notion of computation to digital computation in the face of important yet broader uses of the term "computation" in many sciences, including neuroscience. (BTW, I had this realization in time to dodge Ken Aizawa's criticism that I was insufficiently pluralistic; Ken's criticism does apply to my former self, though.).
After that, I needed to characterize a notion of computation more general than that of digital computation (without appealing to representation, of course, otherwise I would have gone against
one of my core views about computation ).
What came to my rescue is the notion of medium independence. Medium independence was introduced by Justin Garson in his 2003 MA thesis, part of which was published in a beautiful and underappreciated article in Philosophy of Science on "
The Introduction of Information in Neurobiology ".
Justin pointed out that the first person to talk about neural systems transmitting information was Edgar Adrian (1928), on the grounds of his groundbreaking discovery of some crucial properties of neural signals ("all or none", "rate coding", and "adaptation"). Justin reconstructed Adrian's notion of information as involving medium independence:
"Medium independence: The structure S—for example, the structure relation that obtains between the units of a sequence of action potentials—can be instantiated across a wide range of physical mechanisms." (Garson 2003, p. 927)
While Justin's medium independence is not necessary for carrying (natural) information in the usual sense, a slightly modified version of it seems well suited for characterizing the general notion of computation. So in my
more recent work, I characterize computation in the generic sense as (roughly) the functional manipulation of medium independent vehicles according to rules, where a variable is medium independent just in case it is manipulated on the grounds of similarities and differences between its parts along a certain dimension of variation, irrespective of its more concrete physical properties.
Example 1: various sensory receptors transduce all kinds of physical variables into spike trains, which are then conveyed to the nervous system, which in turn manipulates these spike trains. This was one of Adrian's amazing discoveries: neural fibers carry the same kind of signals regarless of their physical inputs. Thus, neural processes are computations in the generic sense.
Example 2: computers manipulate strings of digits, which are well defined so long as there are distinguishable types and an ordering relation, regardless of the details of their physical implementation. The same digital computation can be performed in mechanical, electronic, electromechanical, etc. media. Thus, the activity performed by digital computers are computations in the generic sense (and digital computation is a species of computation in the generic sense).
Example 3: mutatis mutandis, analog computers manipulate their own type of medium independent vehicles. Thus, analog computation is a kind of generic computation.
I hope someone can see the beauty of this. We now have an explicit, non-semantic characterization of computation in general, of which analog computation, digital computation, etc. are species. Thanks Justin!
Anyway, then there is the question of information processing. Obviously information can be "processed" in a medium dependent way, as done by the Watt governors or float regulators that
we've been discussing. If Jonathan insists in calling this type of thing information processing, so be it. But information processing can also be done in a medium independent way, and IMO that's what most people mean when they talk about information processing. In any case, if you care about what does and does not compute it's important to notice the difference between the two cases, because medium-dependent information processing does not entail computation whereas medium-independent information processing does entail computation.