Full Idea
Counterexamples involving prevention and/or interference have come to be roughly divided into four main categories: (i) prevention, (ii) pre-emption, (iii) finks and (iv) antidotes.
Gist of Idea
A cause can fail to produce its normal effect, by prevention, pre-emption, finks or antidotes
Source
R.D. Ingthorsson (A Powerful Particulars View of Causation [2021], 5.3)
Book Reference
'Oxford Handbook of Philosophy of Time', ed/tr. Callender,Craig [OUP 2013], p.90
A Reaction
These are the reasons why necessity is denied in causation. i) is in the initial circumstances, ii) is another cause getting there first, iii) is a defusing action in the agent, iv) is a defusing action in the patient. No necessity if one is possible.