I don’t think that philosophical defenders of theism could ply their trade without employing tu quoque arguments. The use of this device in defense of theistic doctrine goes back at least to Bishop Berkeley. When atheist astronomer Edmund Halley (famous for his comet) charged that the doctrines of Christianity are incomprehensible, Berkeley composed The Analyst which offered the tu quoque argument that the concept of the infinitesimal in Newton’s calculus is as obscure and contradictory as any mystery of Christian theology is said to be. Therefore mathematicians (like Halley) should remove the beams from their own eyes before presuming to admonish Christians about the motes in theirs. More recently, Alvin Plantinga’s whole argument in God and Other Minds is an extended tu quoque. Critics have long complained that there are no good arguments for theism. Plantinga responds with the tu quoque that there is no cogent argument for the existence of other minds, but that surely even the fiercest critics of theism believe that other people have minds. He examines the arguments for other minds and offers what he thinks are knock-down criticisms. His conclusion is that if there is no epistemic sin in believing, in the absence of adequate evidence or argument, that other people have minds, then, the same judgment should be made about theism, that is, that it is rational to believe in God even if that belief is not established by argument or evidence.
Tu quoque arguments definitely have their uses, and though elementary logic texts always stigmatize the tu quoque as a fallacy, I think that we should not proscribe its use entirely. The problem with using tu quoque in the defense of theism is that these arguments often have considerable rhetorical bark but little logical bite. A case in point is an argument given by Sandra Menssen and Thomas D. Sullivan in their recent book The Agnostic Inquirer, a book that I reviewed last April for the on-line Notre Dame Philosophical Review. Menssen and Sullivan say that their book is aimed at unbelievers, the “agnostics” in their title, who honestly inquire about the possibility that there is a good God who has revealed himself to humanity. Much of their book addresses qualms that the “agnostics” are likely to have that could serve a stumbling blocks preventing them from seriously and fairly considering the possibility of divine revelation. One of these likely qualms is the objection that it is inconceivable that a disembodied entity, like God or a Cartesian soul, could causally interact with matter. Here is their argument and my response from the review:
An agnostic inquirer in Menssen and Sullivan’s sense is likely to be, at least, a methodological physicalist, that is, one who, as a matter of methodological or heuristic principle, requires that physical phenomena be explained exclusively in terms of hypotheses postulating physical entities, forces, or processes. One chief motivator of physicalism as a methodological or heuristic principle has been the longstanding and intractable difficulty of understanding how an immaterial entity, like God or a Cartesian mind, could cause a physical effect, e.g., by making a piece of matter move. Menssen and Sullivan admit that we have no idea how mind can move matter (p. 108), and they reply with a tu quoque: We have no idea how matter moves matter. Hence, they imply, it is unreasonable to reject supernatural causal explanations on the grounds that they are less informative than physical ones. With all forms of causality we are stuck where Hume left us, with nothing more than an account of consistent conjunction.
But the point is precisely that with many scientific causal accounts, there is a great wealth of explanatory detail before we reach causal bedrock. Even at the presumably rock-bottom level of quarks, electrons, and photons we have well-confirmed, mathematically precise theories, like quantum electrodynamics, that often make astonishingly accurate predictions. These theories do not just tell us that fundamental particles interact, but give us much information about the way that they do. With supernatural causal explanations, on the other hand, our inquiry simply hits a wall. An advocate of the Cartesian theory of mind can only say that mind does move things by a power which, as Menssen and Sullivan admit, is occult. This is Owen Flanagan’s point when he says that for the Cartesian the mind performs psychokinesis with every voluntary action (Flanagan, 2002, p. 58). Theistic explanations are no better. The evolutionary account of, say, the beginning of mammals is replete with factual and theoretical detail. The creationist story, by contrast, can hardly improve upon the author of the first chapter of Genesis: “God said ‘Let there be…,’ and it was so.” Clearly, compared to the richness of many scientific causal accounts, supernatural causal scenarios are extremely exiguous in content. Therefore, it is not unreasonable for methodological physicalists to demand naturalistic causal accounts.