Re: MacLennan: Grounding Analogue Computers

From: HARNAD Stevan (
Date: Wed Jun 06 2001 - 16:14:45 BST

On Sat, 17 Mar 2001, Hudson Joe wrote:

> 'Grounding Analogue Computers'

> Hudson:
> whatever analogue computation is a digital computer could approximate
> the physical behaviour of the implementation to the nth degree

This just means that a digital computer could simulate any continuous
process to as close an approximation as we wish. But this is still
just simulation (Turing Equivalence, or even Strong Equivalence -- as
close as we like). But, for the same reason that simulated flying (no
matter how closely it approximates it) is not real flying, simulated
continuity is not real continuity.

But never mind continuity. Perhaps it is more instructive to think of
every dynamical physical system, even an airplane, as an "analog device"
of some sort (not necessarily a "computer"). That gives us a better
sense of the gap between the analog and the symbolic (better than
"digital," which focusses too much on just the continuous/discrete

> Hudson:
> MacLennan associates a symbolic view of cognition with a
> discrete one and a connectionist view with an analogue one.

Neural nets can be simulated too, and if the simulations can do the
same internal work that the real parallel distributed ones do, then
the parallel/distributedness is a mere implementational detail, and
connectionism is just a special family of algorithms.

> Hudson
> Just a minute, "assigns meaning...". How does that work? Meaning is
> experienced not 'assigned'.

Correct. Otherwise, it just means "interpretability" (which one can
hardly "assign"!).

> Hudson
> First off how can a state be continuous? State implies something which
> is bounded, so perhaps a sine wave at frequency F1 could be used to
> represent state S1. But then only the representation of the state (the
> sine wave) would be continuous, the state S1 itself would be static and
> discrete. Perhaps MacLennan ment 'the representation of states' rather
> than, "representational states".

No, he just meant states in the usual physical sense, described by
differential equations, hence continuous.

> Hudson:
> Secondly why is transduction a "central issue in symbol grounding"? So
> long as the method of energy conversion from sound pressure, light,
> mechanical resistance, etc. to electrical signals provides enough
> information for the computational bits to function properly why worry
> about it? Isn't the symbol grounding problem more, 'how do we use the
> transduced electrical signals to terminate the hierarchical symbolic
> definition chain?' (But then even if symbols were optimally grounded
> I've no idea how this would conjure up meaning in the system.) Did
> Harnad really say that transduction was the central issue?

Yes he did (if I do say so myself): because sensorimotor transactions
with the objects symbols refer to are the only ones that CANNOT be
symbolic. And chances are, they are part (literally, physically part)
of whatever physical states mental states turn out to be. Hence
mental states, unlike computational states, will not be

So transduction is the most important non-symbolic process, but it's
unlikely to be the only one (as neuropharmacology is showing us).

Stevan Harnad

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