Page 44 - Foundations of Cognitive Psychology : Core Readings

42 Daniel C. Dennett

should resist all limitations and waterings-down of the Turing test. They make
the game too easy—vastly easier than the original test. Hence they lead us
into the risk of overestimating the actual comprehension of the system being
tested.
Consider a different limitation of the Turing test that should strike a suspi-
cious chord in us as soon as we hear it. This is a variation on a theme devel-
oped in an article by Ned Block (1982). Suppose someone were to propose
to restrict the judge to a vocabulary of, say, the 850 words of ‘‘Basic English,’’
and to single-sentence probes—that is ‘‘moves’’—of no more than four words.
Moreover, contestants must respond to these probes with no more than four
words per move, and a test may involve no more than forty questions.
Is this an innocent variation on Turing’s original test? These restrictions
would make the imitation game clearly finite. That is, the total number of all
possible permissible games is a large, but finite, number. One might suspect
that such a limitation would permit the trickster simply to store, in alphabetical
order, all the possible good conversations within the limits and beat the judge
with nothing more sophisticated than a system of table lookup. In fact, that
isn’t in the cards. Even with these severe and improbable and suspicious
restrictions imposed upon the imitation game, the number of legal games,
though finite, is mind-bogglingly large. I haven’t bothered trying to calculate it,
but it surely exceeds astronomically the number of possible chess games with
no more than forty moves, and that number has been calculated. John Hauge-
land says it’s in the neighborhood of ten to the one hundred twentieth power.
For comparison, Haugeland (1981, p. 16) suggests that there have only been ten
to the eighteenth seconds since the beginning of the universe.
Of course, the number of good, sensible conversations under these limits is a
tiny fraction, maybe one quadrillionth, of the number of merely grammatically
well formed conversations. So let’s say, to be very conservative, that there are
only ten to the fiftieth different smart conversations such a computer would
have to store. Well, the task shouldn’t take more than a few trillion years—
given generous government support. Finite numbers can be very large.
So though we needn’t worry that this particular trick of storing all the smart
conversations would work, we can appreciate that there are lots of ways of
making the task easier that may appear innocent at first. We also get a re-
assuring measure of just how severe the unrestricted Turing test is by reflect-
ing on the more than astronomical size of even that severely restricted version
of it.
Block’s imagined—and utterly impossible—program exhibits the dreaded
feature known in computer science circles as combinatorial explosion.Noconcei-
vable computer could overpower a combinatorial explosion with sheer speed
and size. Since the problem areas addressed by artificial intelligence are verita-
ble minefields of combinatorial explosion, and since it has often proven difficult
to find any solution to a problem that avoids them, there is considerable plau-
sibility in Newell and Simon’s proposal that avoiding combinatorial explosion
(by any means at all) be viewed as one of the hallmarks of intelligence.
Our brains are millions of times bigger than the brains of gnats, but they are
still, for all their vast complexity, compact, efficient, timely organs that some-
how or other manage to perform all their tasks while avoiding combinatorial