adverbs

If John swam fast, then it follows that John swam. But how do we understand this inference? In first-order logic, the natural suggestion is that there is an individual, an event which was a swimming by John, and was fast. But now suppose John's swimming was also a crossing, and although John swam fast his crossing was not fast (most people cross by much faster means). Then the one event was a swimming and a crossing, and fast and not fast, which sounds like a contradiction. The question for logic and semantic theory is whether adverbs are to be regarded as predicates of speicial individuals such as events, or as predicate modifiers.