Full Idea
If laws of causation are probabilistic then the law does not entail any restrictions upon the proportion of events that follow a cause: ...it can have absolutely any value from zero to one.
Gist of Idea
Probabilist laws are compatible with effects always or never happening
Source
Michael Tooley (Causation and Supervenience [2003], 4.1.3)
Book Reference
'The Oxford Handbook of Metaphysics', ed/tr. Loux,M /Zimmerman,D [OUP 2005], p.396
A Reaction
This objection applies to an account of laws of nature, and also to definitions of causes as events which increase probabilities. One needn't be fully committed to natural necessity, but it must form some part of the account.