of English-"the", "of", "and", "a", and "to"-would be equivalent to
solving the entire problem of AI, and hence tantamount to knowing what
intelligence and consciousness are. A small digression: the five most com-
mon nouns in English are-according to the Word Frequency Book compiled
by John B. Carroll et al-"time", "people", "way", "water", and "words" (in
that order). The amazing thing about this is that most people have no idea
that we think in such abstract terms. Ask your friends, and 10 to 1 they'll
guess such words as "man", "house", "car", "dog", and "money". And--
while we're on the subject of frequencies-the top twelve letters in English,
in order, according to Mergenthaler, are: "ETAOIN SHRDLU".
One amusing feature of SHRDLU which runs totally against the
stereotype of computers as "number crunchers" is this fact, pointed out by
Winograd: "Our system does not accept numbers in numeric form, and has
only been taught to count to ten."19 With all its mathematical underpin-
ning, SHRDLU is a mathematical ignoramus! Just like Aunt Hillary,
SHRDLU doesn't know anything about the lower levels which make it up.
Its knowledge is largely procedural (see particularly the remark by "Dr.
Tony Earrwig" in section 11 of the previous Dialogue).
It is interesting to contrast the procedural embedding of knowledge in
SHRDLU with the knowledge in my sentence-generation program. AU of
the syntactical knowledge in my program was procedurally embedded in
Augmented Transition Networks, written in the language Algol; but the
semantic knowledge-the information about semantic class
membership-was static: it was contained in a short list of numbers after
each word. There were a few words, such as the auxiliary verbs "to be", "to
have", and others, which were represented totally in procedures in Algol,
but they were the exceptions. By contrast, in SHRDLU, all words were
represented as programs. Here is a case which demonstrates that, despite
the theoretical equivalence of data and programs, in practice the choice of
one over the other has major consequences.
Syntax and SemanticsAnd now, a few more words from Winograd:Our program does not operate by first parsing a sentence, then doing seman-
tic analysis, and finally by using deduction to produce a response. These three
activities go on concurrently throughout the understanding of a sentence. As
soon as a piece of syntactic structure begins to take shape, a semantic program
is called to see whether it might make sense, and the resultant answer can
direct the parsing. In deciding whether it makes sense, the semantic routine
may call deductive processes and ask questions about the real world. As an
example, in sentence 34 of the Dialogue ("Put the blue pyramid on the block
in the box"), the parser first comes up with "the blue pyramid on the block" as
a candidate for a noun group. At this point, semantic analysis is done, and
since "the" is definite, a check is made in the data base for the object being
referred to. When no such object is found, the parsing is redirected to find
the noun group "the blue pyramid". It will then go on to find "on the block in(^630) Artificial Intelligence: Retrospects