The necessitarian states that there are metaphysical connections of necessity in the world that ground and explain the most fundamental regularities. Necessitarian theorists usually use the word must to express this connection. Thus, NT maintains must-statements are not adequately captured by is-statements (must ≠ is, or certain facts are unaccounted for). Nomic necessity claims that it is difficult for mere regularity to account for certain counterfactual claims because what happens in the actual world do not themselves imply anything about what would have happened had things been different. If it is now true that Q occurs if P causally precedes Q then the necessitarian can adequately account for counterfactual claims. Given the present antecedent condition of P at tn and P implies Q at tn and it was true that P implied Q at tn-1 then using P as an antecedent for R at hypothetical tn-1’ then R is true if P was a sufficient condition R at tn-1’. Thus, there is certainty in the truth of counterfactual claims. However, counterfactuals allow for conflict between truth functional interpretation and ordinary language. For instance, any counterfactual claim with the necessary condition having a false truth-value and the sufficient condition obtaining a truth-value that is true then the counterfactual claim will be invalid.
Why is supporting counterfactual conditionals a symptom of nomic necessity? I would ague that there must be a connection or relationship between the conditions. Consider the argument, as modus ponens, that if the moon’s core is made of cheese then my desk is made from mahogany. What relationship do these two conditions have?