Abstract I argue that the special behavior of adjuncts is a consequence of two properties that set them apart from arguments: optionality and independence.

• Optionality Adjuncts can be omitted.
• Independence Independently well-formed adjuncts can be combined.

These properties yield several grammaticality inferences that mirror the entailments of the logical connector and. For instance, just like t = 0 implies t & a = 0 for propositions, the ungrammaticality of tree t entails that the result of adding adjunct a to t is also ungrammatical. Intuitively, these grammaticality entailments render adjuncts semi-permeable with respect to constraints —- dependencies can “scope out” of adjuncts and thus restrict the shape of the rest of the tree, but not the other way round. In combination with standard assumptions about the feature-driven nature of Move, semi-permeability derives the Adjunct Island Constraint while still allowing for parasitic gaps.