This paper defends a principle-based model of sen-
tence processing, and demonstrates that such a model
must have two specific properties: (1) it must use a
partially top-down parsing mechanism, possibly re-
stricted to functional structure, and (2) it must use
an Active Trace Strategy, which freely posits traces
before their linear string position. It is argued that
both of the proposed mechanisms follow from an over-
arching Principle of Incremental Comprehension.
