Harnad responds

1. For me, obeying differential equations is physics and implementing symbol manipulations is computation. They are not the same kind of thing, and indeed, the difference between them, the hardware/software distinction, is what most of this discussion is about. (Nor, as far as I can see, does this have anything to do with the psychologists' hoary example (which is often cited to illustrate one insight or another, but has not yet led to any concrete results) that the dog (somehow) correctly estimates the trajectory of the frisby without doing a conscious calculation.)
2. Nor, from the equally venerable platitude that continuous signals can be approximated arbitrarily closely by discrete ones, does it follow that virtual objects are approximately real: Tighten the approximation as close as you like, a simulated waterfall will never be wet. Roitblat seems to conflate discrete approximation of continuous signals -- a transducer function -- with systematically interpretable symbolic representation (see Harnad 1987).
3. Roitblat seems to think grounding has to do with ``specifying the premises of formal arguments.'' I cannot see this it all; it has to do with connecting symbols to what they are about by means other than an external interpretation: my candidate is causal grounding via whatever internal resources it takes to pass the TTT.
4. Roitblat seems to be using the words ``syntax,'' ``systematicity,'' and and especially ``semantics'' very differently from the way I do. I think my usage is more standard (e.g., see Fodor & Pylyshyn 1988), but let the reader judge.
5. We also differ in our construal of the Chinese Gym and Chinese Room Arguments, for which I can only refer the reader to the target article and the original sources (Searle 1980, Harnad 1989 Harnad 1990a) for comparison.