Full Idea
An account of events just in terms of physical bodies does not distinguish between events that happen to take up just the same portion of space-time. A man's whistling and walking would be identified with the same temporal segment of the man.
Gist of Idea
Explaining events just by bodies can't explain two events identical in space-time
Source
Willard Quine (On Multiplying Entities [1974], p.260)
Book Reference
Quine,Willard: 'Ways of Paradox and other essays' [Harvard 1976], p.260
A Reaction
We wouldn't want to make his 'walking' and his 'strolling' two events. Whistling and walking are different because different objects are involved (lips and legs). Hence a man is not (ontologically) a single object.