Natural law theory holds that a large and substantive body of moral knowledge can be had apart from divine revelation. Natural theology holds that a large and substantive body of theological knowledge can be had apart from divine revelation. Yet both secular and religious critics of natural law theory and natural theology sometimes accuse them of smuggling in the deliverances of revelation. For example, theologian David Bentley Hart, in his recent attacks on natural law theory (to which I responded here, here, and here), seemed to take the view that natural law arguments implicitly presuppose revealed or supernatural truths. Secular critics routinely accuse natural law theorists of rationalizing conclusions that they would never have arrived at if not for the teachings of the Bible or the Church. Critics of the Scholastic tradition in philosophy sometimes accuse it of constructing metaphysical notions ad hoc, for the sake of advancing theological claims. (My friend Bill Vallicella has made this complaint vis-à-vis the Scholastic notion of suppositum.) In every case the objection is that if an idea has an origin in a purported source of divine revelation, its status as a purely philosophical thesis or argument is ipso facto suspect.
One of the problems with such objections is that they overlook the distinction between what Hans Reichenbach called the “context of discovery” and the “context of justification” -- a distinction he applied within the philosophy of science, but which has application in other contexts too.
Back to the Future fans will recall that Doc Brown hit upon the idea of the flux capacitor(which, of course, makes time travel possible) after slipping on the wet porcelain of his toilet while hanging a clock and banging his head on the sink. Presumably, however, since the flux capacitor worked, he had independent reason -- actual scientific arguments -- for thinking that it would work. That he had a flash of insight as a result of a blow to the head may have prompted the discovery of the flux capacitor, but it wasn’t his justification for believing that it would work. He wouldn’t have said: “I had a sudden flash of inspiration as a result of hitting my head; therefore the flux capacitor will work!”
Of course the example is just for laughs, but the serious point behind it is that there is a crucial difference between, on the one hand, the psychological, social, cultural, or historical factors that led to an idea, and, on the other hand, its epistemological or logical credentials. A physicist might hit upon a solution to the problem he is working on while listening to Bach, reading a comic book, or soaking in the tub. Something in those particular circumstances might trigger the crucial thought. It doesn’t follow that the worked-out solution, though packaged and defended in the way arguments in physics typically are, is “really” grounded in music theory, panelology, or plumbing. It could even be that certain theories in physics were in practice extremely unlikely to have been discovered in the absence of certain crucial historical, cultural, and economic factors. It still simply doesn’t follow that physics “really” boils down to history, culture, or economics. The evidence and arguments for a scientific theory can be evaluated apart from the economic interests its discovery served, the cultural attitudes it fosters, or the historical period it reflects.
Those who suppose otherwise, claiming that the epistemological reduces to the psychological and sociological, inevitably refute themselves. For either their own position itself constitutes just the sort of extra-psychological and extra-sociological point of view they told us was not possible, or it reduces to just one psychologically and sociologically relative position among others, with no better claim to our allegiance than the views it criticizes. (See pp. 46-49 of The Last Superstition for more on this sort of problem with relativism.)
Now, a similar point holds for natural law and natural theology and their relationship to particular religious traditions. That a certain moral or theological proposition or argument arose in (say) a Christian context, even if it was extremely unlikely to have arisen outside that context, simply does not by itself show that it is impossible in principle to justify via philosophical arguments that make no reference to sources of Christian revelation. Suppose, for example, that we accept the common view that Aristotle did not regard the Unmoved Mover as the cause of the world’s being sustained in existence, but only of its motion. (That is sometimes disputed, but if you doubt it just suppose it is the case for the sake of argument.) It simply does not follow that God’s continual creation of the world out of nothing can’t be defended on Aristotelian premises -- even if Aristotle himself did not see this, and even if Christian thinkers were motivated to look for arguments to this effect because of their prior commitment to revelation.
Atheists who are happy to accept Newton’s scientific ideas despite his theological commitments, or Martin Luther King’s moral ideas despite his theological commitments, are being inconsistent if they glibly dismiss Aquinas’s arguments in natural theology or natural law as mere rationalizations of what he already believed on the basis of revealed theology. Theologians who deny that there is any theological or moral knowledge to be had apart from special revelation are playing into the hands of such atheists, implicitly stripping the faith of the philosophical preambles Aquinas and Catholic teaching more generally recognize that it needs. In any event, the genetic fallacy remains a fallacy even when deployed against ideas you don’t like. Or to exaggerate the significance of ideas you do like.
(I can just see a certain kind of continentally-trained theologian or religious philosopher reading this post and concluding: “A-ha! I knewFeser must be some kind of closet positivist! He just cited Reichenbach -- that clinches it!” That would, of course, itself be an instance of the very fallacy I’m criticizing. That an idea arose in a positivist context simply does not entail that it is an inherently positivist idea.)