Full Idea
According to Perdurance Theory, it is long-lived sums of stages which are tennis balls, whereas according to Stage Theory, it is the stages themselves which are tennis balls.
Gist of Idea
Perdurance says things are sums of stages; Stage Theory says each stage is the thing
Source
Katherine Hawley (How Things Persist [2001], 2.2)
Book Reference
Hawley,Katherine: 'How Things Persist' [OUP 2004], p.42
A Reaction
These seem to be the two options if you are a four-dimensionalist, though Fine says you could be a weird three-dimensionalist and choose stage theory.
Related Idea
Idea 12279 The differentia indicate the qualities, but not the essence [Aristotle]