The Lexicalization Hypothesis is a hypothesis proposed by Chomsky that states that syntactic transformations can only apply on syntatic constituents; therefore, the rules of putting words together is different from the rules that puts phrases together. This theory stands in opposition to generative semantics.
There are two versions of the Lexicalization Hypothesis:
Strong Lexicalization Hypothesis
The Strong Lexicalization Hypothesis states that both derivational words (changes meaning, bench=>benching) or inflectional words (changes grammar, eat=>eating) cannot be put together via syntatical rules. (Geeraerts 2009)
Weak Lexicalization Hypothesis
Weak Lexicalization Hypothesis states that semantic rules cannot work in the formation of derivational words only.