role it plays in children's getting rid of over-regularizations. I then show how this
principle helps account for variations in the courses children follow during ac-
quisition. In the last section I argue that this principle subsumes several other
proposals to constrain language development in that they each constitute special
cases of the Principle of Contrast.

The Principle of Contrast states that any difference in form in a language marks a
difference in meaning. The term dog, for instance, which differs in form from
horse also differs from it in meaning. This principle can be stated as:

The Principle of Contrast: Every two forms contrast in meaning.

This principle is a general one for speakers of a language. It is one that has been
stated or assumed by virtually every linguist over the years.

The Principle of Contrast must be carefully distinguished from its converse,
which I will call the Homonymy Assumption. This assumption is that every two
meanings contrast in form. Under this view, one should never find two different
meanings being carried by the same form, as in bank of a river versus a financial
institution, or bat, a small flying mammal versus an instrument used in playing
cricket or baseball. This assumption clearly doesn't hold in general for speakers
of a language. But, within one level of a semantic field, where the words for two
different meanings over time come to have the same form, the resultant hom-
onymy may cause genuine confusion. Speakers then typically introduce another
form to carry one of the meanings (see Orr, 1962). Aside from this special case,
the Homonymy Assumption should be kept distinct from the Principle of Con-
trast because it may play little or no role in either adult language use or
acquisition.

The Principle of Contrast is essential, though, because it helps maintain
conventionality in language:

The Principle of Conventionality: For certain meanings, there is a conventional
form that speakers expect to be used in the language community.

If one wishes to talk about an instance of the category 'dog', one had better use
the conventional word dog (and not horse), or no one will understand. Conven-
tional terms used conventionally work best to convey speakers' intentions within
the speech community. Conventional terms work in large part because speakers
are consistent with the conventional meanings they assign to forms from one
occasion to the next, and therefore maintain the same contrasts in meaning over

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