Gerald M. Weinberg: An Introduction to General Systems Thinking ------------------------------------------- Chapter 3: System and Illusion Section: A System Is a Set Pages 65/66 Computers have ways of exposing flaws in explicit rules. In the attempt to mechanize a classification procedure, we usually discover much more than meets the eye. Cytologists have long been able to select slides of cells with ``abnormal'' chromosomes; lawyers have always been able to choose precedents ``relevant'' to a case; and grammarians never had much difficulty classifying sentences according to their ``structure''. But when they tried to mechanize -- to make the rules explicit enought for a computer -- cytologists, lawyers, and grammarians discovered that they never knew precisely what they were doing. Everyone is familiar with classifying sentences according to grammatical structure. In this area, one of the classic computer examples is the sentence: TIME FLIES LIKE AN ARROW. Few of us have difficulty recognizing the structure of this sentence: TIME is the subject, FLIES is the verb, LIKE AN ARROW is the predicate. This *seems* to be a purely *grammatical* analysis. It uses only parts of speech, and not the probable *meanings* of the words, which would make it a *semantic* analysis. When using the computer, things are not so easy: TIME may be a noun, but it may also be an adjective, as in TIME CLOCK. FLIES may be a verb, but it could also be a noun, as in FRUIT FLIES. LIKE may be a preposition, but it may also be a verb, as in I LIKE YOU. Given these possibilities, how do we know that the structure of TIME FLIES LIKE AN ARROW. is not the same as the structure of FRUIT FLIES LIKE A BANANA. The answer is that we do not know. We jumped to a conclusion based on a probable *semantic* interpretation. If the sentence had been FRUIT FLIES LIKE AN ARROW. we might have more easily recognized the ambiguity. Instead, this parsing ``emerges'' from the computer. Initially, we thought this classification involved only grammatical considerations, but it went much deeper. We were unconscious of the choice process taking place in our own mind, yet even when we feel aware of possible ambiguities, more may be lurking in the shadows. As the computer revealed, there is yet another prefectly *grammatical* interpretation of TIME FLIES LIKE AN ARROW. in which TIME is a verb, and the sentence is imperative, analogous to the form of the sentence TIME RACES LIKE A TIMEKEEPER. Without the computer to keep us on our toes, we would remain sloppy grammarians, but unaware of our sloppiness.