Jonathan Schaffer is famous for his Priority Monism, or (metaphysical) Monism in general. He has worked on Fundamentality, Truthmaking and Causation among others. In 'Monism. The Priority of the Whole, he accepts the possibility of gunk. Is it known what exactly his view on modality is?
We have some idea of his views on modal necessitiarianism in regard to laws of nature. Schaffer distinguishes three different forms of such necessitarianism (Alastair Wilson, 'Schaffer on laws of nature',Philosophical Studies: An International Journal for Philosophy in the Analytic Tradition, Vol. 164, No. 3 (July 2013), p. 654) :
▻ Modal necessitarianism
Modal necessitarianism The actual laws are the laws of all possible worlds.
Nomic necessitarianism Properties are governed by the same laws in all worlds in which they are instantiated.
Causal necessitarianism Properties have the same causal roles in all worlds in which they are instantiated.
In his attack on modal necessitarianism he critiques two arguments :
▻ The argument from natural necessity
(1) If the relation between properties and their powers is contingent, then like charges might not repel;
(2) Like charges must repel;
(3) Therefore: the relation between properties and their powers is not contingent.
He argues that there is an equivocation on the modal strengths of 'might not' and must' (Wilson, p.656).
▻ The argument from sustaining counterfactuals
(4) If the relation between properties and their powers is contingent, then there is nothing that guarantees that like charges repel in any other possible world;
(5) In the nearest possible world, like charges repel;
(6) Therefore the relation between properties and their powers is not contingent.
What does he object to here ?
Schaffer's response to the argument from sustaining counterfactuals is to deny that the consequent of (4) is inconsistent with (5). He contends that even if charges repel in some possible worlds, there can be a guarantee that charges repel in the nearest possible world if fixity of laws is partly constitutive of nearness, as is the case according to the influential set of criteria for nearness of worlds set out by Lewis (1979). (Wilson, p.658.)
Lewis, D. (1979). 'Counterfactual dependence and time's arrow.' Nous, 13/4, 455-476.