Full Idea
'The ball's path is determined' must mean 'there is only one possible path for the ball (assuming no air currents)', but what ground could one have for believing this, if one does not believe in some system for which it is a consequence?
Gist of Idea
To believe in determinism, one must believe in a system which determines events
Source
G.E.M. Anscombe (Causality and Determinism [1971], §2)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.97
A Reaction
This seems right, but it doesn't follow that one has to know the full details of the system. The system might just be the best explanation, or even a matter of vague faith. It might, though, be just that you can't imagine any other outcome.