Full Idea
Each volition can neither exist nor be determined to produce an effect unless it is determined by another cause, and this cause again by another, and so on, to infinity.
Gist of Idea
An act of will can only occur if it has been caused, which implies a regress of causes
Source
Baruch de Spinoza (The Ethics [1675], I Pr 32), quoted by Stephan Schmid - Faculties in Early Modern Philosophy 3
Book Reference
'The Faculties: a history', ed/tr. Perler,Dominic [OUP 2015], p.168
A Reaction
Acts of will are usually responses to situations, so it seems a bit simplistic to think that they are all spontaneous sui generis causal events. That argument won't work, of course, for a random volition that is out of context.