[This is a reply to a commenter at Dr Feser's blog. The comments were having so many glitches that, once I glued my hair back into my scalp, I decided to post it here. Effing technology.]
First of all, the very ability to make false predictions, and then correct them, is proof that science works with contingency. A necessitarian cannot coherently say both "Science shows X is true" and "I might be wrong that X". As a devotee of the scientific method, you admit this or that scientific claim can be falsified––well, there's more contingency for you. Or are you really a full-blown fatalist?
Second, specific quantities are inherently contingent: it is not an a priori truth that the atomic weight of carbon is 12.01 (if it is, of course, that's no longer a scientific claim but a Platonic demonstration in the making). What is of-a-specific-amount could have been, and could be, of a different amount, by the very notion of what "specific amount" means. Only an infinite object is free of quantitative contingency: the quantitatively specific is what it is, and not something else, and is therefore confined to its proper, contingent features. The act of measuring implies there is a contingency in our knowledge of the object, otherwise there would be no need to measure. If you reply this contingency is merely due to our ignorance of otherwise necessary circumstances, you still grant the point by admitting at least the contingency of our knowledge/ignorance.
Third, radioactive decay (emission rates, exactly which atoms get emitted, etc.) and quantum indeterminacy are inherently contingent. Along the same lines, nearly all evolutionists agree a "second run" of terrestrial evolution would not result in the world as we now know it, even despite metaphysical necessity. Moreover, the notion of evolution itself demands contingency, for a necessary change is incoherent. Why? Because insofar as the state of affairs named by that change (SOA:c) includes the conditions for its coming to be, SOA:c can not not-exist, otherwise its conditions for being would not in fact be necessary. That which includes absolutely necessaryr conditions of being exists from all time, and therefore cannot come to be from not-being. Evolution literally cannot happen if, necessarily, only what happens can happen. That's called contingency, my neo-Darwinian friend.
Fourth, let's not forget Gödelian incompleteness and Turing-Church undecidability. There's contingency even in them thar "formal" hills you love so much.
Fifth, there is the contingency of our competing claims in this very exchange. You claim D: that contingency is unverifiable and probably even unreal. I claim C: that contingency is real and evident. One of us is right. And yet which of us is right is intrinsically contingent (obviously so if C is granted even without the point I'm making now). Even were D true, however, it would still only be so contingently. Why? Because there is no analytic, apodeictic necessity in D. It is not an identity statement, nor is it manifestly "evident to the senses" (otherwise we couldn't even be having this dispute). Insofar as it's up for debate, the truth of D is contingent, since truth only exists in a mind, even if the truthmaker of D is not contingent.
Now, only if you mount a radically actualist ontology (á la A: necessarily, whatever is, is, and whatever is, necessarily is, since only that which is can possibly be)––only on such a teratological Parmenidian-Meinongian hybrid could you defend thoroughgoing necessitarianism of the kind needed to eliminate contingency even as a meaningful category. If you were to mount A, however, you would commit yourself to death by "Ockham's" razor and suffocation in a grotesque Borgesian ontology (have fun barbering Plato's beard). You're not a philosopher by trade, so you might not be aware that it's actually necessity which has taken more of a beating than possibility in the last century or three of analysis (Scotus, Ockham, Hume, Sartre, Quine, et al.). Even if you accept a Kripkean essentialism, with its attendant modes of necessity, you still only have the resources for subsequent necessity, not antecedent necessity per se.
Finally, my point about the fallacy of affirming the consequent and evolutionary science is that your confidence in the latter is vitiated by your own claim that "Any logically invalid argument is always logically invalid, even when it is accidentally true." Ergo, evolution is logically invalid. You're being parsimonious with your logicism. You've admitted you come here to be antagonistic. You have the stink of sophistry about you in more than one pair of nostrils. You have five children and a wife: perhaps more time with them and less time online pedaling ineffectual and seemingly willful pedantry would be for the good.