Full Idea
The relationship between co-existing properties or successive events or states is naturally necessary when they are understood by scientists to be related in fact by generative mechanisms, whose structures constitute the essential nature of things.
Gist of Idea
Relations are naturally necessary when they are generated by the essential mechanisms of the world
Source
S.Mumford/R.Lill Anjum (Getting Causes from Powers [2011], 7.3)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.130
A Reaction
This is the view I espouse. It doesn't follow that those mechanisms have necessary existence. Given those mechanisms, they can only behave in that way, because behaving in some way is precisely what they are.