Full Idea
When the natures of the operative powerful particulars, the constraining or stimulating effect of conditions and so on are offered as the grounds for the judgement that a certain effect cannot but happen (or fail), we have natural necessity.
Gist of Idea
Natural necessity is when powerful particulars must produce certain results in a situation
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 1.V.B)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.20
A Reaction
This is the view I subscribe to, the really right bit of scientific essentialism. Can this view be proved? Hm. I take the opposite view to be the misguided Humean idea that if you can imagine it not happening, then it might not happen. Firey furnace.