The disagreement is over what counts as computation, I think that Harnad and I both agree that the dog solves the trajectory problem implicitly. This definition is important, because, although Harnad offers a technical definition for what he means by computation, the folk- definition of the term is probably interpreted differently, and I believe this leads to trouble. I think that this claim stems in part from his definition of computation as based on arbitrary symbol manipulation. If one accepts that computers can perform operations other than pure symbol manipulation, then I think, the distinction evaporates. So-called symbolic systems can represent analog properties along various dimensions with arbitrary precision.
Watches are an example of a system that exists in both digital and analog form.
The position of the hour hand, for example, revolves exactly twice for each rotation of the earth. It preserves continuity between times in continuity between positions. A digital watch also represents the passage of time, but there are no analogs between the parts of the digital display and the passage of time. Symbols appear twice during each 24 hours. The symbols depend on the time of day, but the similarity between two displays does not correspond to the similarity between the two times.