Separation hypothesis

Definition
In morphological theory, the Separation Hypothesis (originally formulated by Robert Beard) is the claim that that the form of inflectional and derivational affixes is separated from their function. Beard distinguishes L-rules and M-rules, and assumes that L-rules are grammatical processes which change or add information about grammatical functions (e.g. 'plural' or 'agent noun'), while M-rules are affixation rules which spell out the grammatical functions.

Example
English plurals are formed in a number of ways, as is shown in (i):

(i) cat-cats, bus-busses, alga-algae, paramecium-paramecia, goose-geese

Under the separation hypothesis there is a single L-rule of pluralization which simply adds the feature [plural]. The resulting abstract morpheme is input to different M-rules, and these rules spell out the actual phonological form of the plurals in (i). On the other hand, conversion can be seen to be simply the situation which arises when an L-rule applies, but no M-rule gets the chance of giving phonological content to the function supplied by the L-rule.

Links
Utrecht Lexicon of Linguistics