Full Idea
The two alternatives to volitionism in explaining action are (firstly) certain complexes of belief and desire, and (secondly) causation by an agent.
Clarification
'Volitionism' says the will is the explanation of action
Gist of Idea
The three main theories of action involve the will, or belief-plus-desire, or an agent
Source
E.J. Lowe (Introduction to the Philosophy of Mind [2000], Ch. 9)
Book Reference
Lowe,E.J.: 'Introduction to the Philosophy of Mind' [CUP 2000], p.250
A Reaction
A helpful framework. A key test case seems to that of trying to perform an action and failing (e.g. through paralysis), and this goes against the whole 'agent' being the most basic concept. One also needs room for reasons, and this supports volitionism.