Problem 8.3 All of the constraints are necessary. None follows from the others.

1. Structural licensing refers to a different property of the formalism than matching or biuniqueness and so cannot be derived from them nor they from it. Structural licensing defines the phrase structure configuration in which case licensing occurs, while biuniqueness refers to the number of cases that a head can check and matching refers to which particular cases a head can check.
2. Biuniqueness and matching are clearly independent of each other as well. Without biuniqueness, two different arguments of a single head could be checked by a single Case feature of that head. Without matching, the actual values of Case features would be irrelevant, since there would be no mechanism to distinguish different Case features from each other.
3. Exocentricity is needed to keep a determiner from assigning case to its own DP maximal projection. Without exocentricity, DP's could appear where no external case assigner was present, for example as the subject of any infinitive. None of the other constraints prevent this. Exocentricity can be thought of as a structural licensing condition, but it is not reduceable to the condition in (43).

Biuniqueness cannot be derived from structural licensing and exocentricity combined because the addition of the latter to the former doesn't affect the relevant cases, For example, without biuniqueness a verb, head of VP, could license accusative case on both its specifier and its complement, under the Head-Spec and Head-Complement configurations, respectively. Then DP's could appear freely as subjects of non-finite transitive verbs.