This paper investigates the relationship between variability
and lexicality on the one
hand and sound change on the other within the theory of Lexical Phonology.
The
former leads to the proposal of the Optionality Constraint (OC), which
prohibits the
application of optional rules in the lexical module. The constraint is
found to be
violated at the word level. The violation of OC as well as of other lexical
modular
principles is accounted for by the help of a new licensing
principle, called the Polarity
Principle. This allows for interacting modules to have different properties
of
representation and rule application at their opposite ends. The OC leads
to a
resolution of the Neogrammarian Controversy, that is consonant with the
standard
assumption concerning sound change, namely, the inherent relation between
the
latter and variability.