(Title quote from Victor J. Stenger, Quantum Gods, Amherst: Prometheus, 2009, p. 159).
According to C. S. Lewis, "[t]he shortest and simplest form of [the Argument from Reason] is that given by Professor J. B. S. Haldane…. He writes, 'If my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose that my beliefs are true … and hence I have no reason for supposing my brain to be composed of atoms'" (Miracles: A Preliminary Study, London: Geoffrey Bles, 1947, pp. 28 & 29). I am quoting from the original version of Miracles, but, as G. E. M. Anscombe noticed, "The last five pages of the old chapter have been replaced by ten pages of the new, though a quotation from J. B. S. Haldane is common to both" (The Collected Philosophical Papers of G. E. M. Anscombe Vol. II, Metaphysics and the Philosophy of Mind, Basil Blackwell, 1981, p. ix, "Appendices to 'What Lewis Really Did to Miracles," Journal of Inklings Studies I, 2, October 2011). As you probably noticed, there are ellipses in the Haldane quote. I was curious to know what Haldane's reasoning might have been. Here is the full quote: "It seems to me immensely unlikely that mind is a mere by-product of matter. For if my mental processes are determined wholly by the motions of atoms in my brain I have no reason to suppose that my beliefs are true. They may be sound chemically, but that does not make them sound logically. And hence I have no reason for supposing my brain to be composed of atoms" (Possible Worlds, London: Chatto and Windus, 1930, p. 209). The full quote is essential because here Haldane brings up the issue of soundness. According to John Beversluis,
To be sound, an argument must be valid and its premises must be true. In that sense, validity and truth are logically independent and determined in different ways. Whether an argument is sound or unsound depends on whether its premises are true or false. That, in turn, depends on the way the world is, on the facts. But whether an argument is valid or invalid depends completely on its logical form. (C. S. Lewis and the Search for Rational Religion Revised and Updated, Amherst: Prometheus, 2007, p. 186)
Also,
The conclusion of a valid deductive argument has nothing to do with the truth of the statements it contains. A deductive argument can be valid even if all the statements it contains are false―valid but not sound. To say that an argument is sound is to make a different claim. A sound argument is not only valid; it is an argument whose statements are true…. [V]alidity and truth are determined in wholly different ways. Knowledge (understood as truth) simply does not depend on validity. (C. S. Lewis and the Search for Rational Religion, Grand Rapids: Eerdman's, 1985, pp. 75 & 76)
I believe that Haldane was confused about the distinction between true premises and theorems. Logical soundness only applies to theorems, but soundness can apply to any true belief. As Beversluis explained, "It is only because we already know some truths about the world that we can employ our 'powers' of reasoning to deduce other truths from them." The way I read Beversluis, he distinguished between "truths about the world" and "truths". The former, in this context, meant "true premises". Given my interpretation, I think I can agree with Beversluis that "it is experience, not reason, that provides us with truths about the world" (C. S. Lewis and the Search for Rational Religion p. 77). (There may be some exceptions in the cases of true axioms.) The proposition that your brain is composed of atoms need not be deduced logically from a true premise. The proposition about the composition of a brain or anything must be tested empirically. In short, logical accuracy (validity) and empirically accuracy (truth) must only be present for a sound deduction. A true belief (premise), on the other hand, must only be empirically accurate. At the risk of taking him out of context, I'll add that even Lewis himself wrote, "Reason knows that she cannot work without materials. When it becomes clear that you cannot find out by reasoning whether the cat is in the linen-cupboard, it is Reason herself who whispers, 'Go and look. This is not my job: it is a matter of the senses'" (Miracles: A Preliminary Study, London: Geoffrey Bles, 1947, p. 110; Revised, HarperCollins, 2001, p. 144).
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