Dennett (1995) and th modal falacy

“In the “weak form,” it is a sound, harmless, and on occasion useful application of elementary logic: if x is a necessary condition for the existence of y, and y exists, then x exists. If consciousness depends on complex physical structures, and complex structures depend on large molecules composed of elements heavier than hydrogen and helium, then, since we are conscious, the world must contain such elements.

But notice that there is a loose cannon on the deck in the previous sentence: the wandering “must.” I have followed the common practice in ordinary English of couching a claim of necessity in a technically incorrect way. As any student in logic class soon learns, what I really should have written is:

It must be the case that: if consciousness depends … then, since we are conscious, the world contains such elements.

The conclusion that can be validly drawn is only that the world does contain such elements, not that it had to contain such elements. It has to contain such elements for us to exist, we may grant, but it might not have contained such elements, and if that had been the case, we wouldn’t be here to be dismayed. It’s as simple as that.

Some attempts to define and defend a “strong form” of the Anthropic Principle strive to justify the late location of the “must” as not casual expression but a conclusion about the way the universe necessarily is. I admit that I find it hard to believe that so much confusion and controversy are actually generated by a simple mistake of logic, but the evidence is really quite strong that this is often the case, and not just in discussions of the Anthropic Principle. Consider the related confusions that surround Darwinian deduction in general. Darwin deduces that human beings must have evolved from a common ancestor of the chimpanzee, or that all life must have arisen from a single beginning, and some people, unaccountably, take these deductions as claims that human beings are somehow a necessary product of evolution, or that life is a necessary feature of our planet, but nothing of the kind follows from Darwin’s deductions properly construed. What must be the case is not that we are here, but that since we are here, we evolved from primates. Suppose John is a bachelor. Then he must be single, right? (That’s a truth of logic.) Poor John—he can never get married! The fallacy is obvious in this example, and it is worth keeping it in the back of your mind as a template to compare other arguments with.” (p. 165-166, Darwin’s Dangerous Idea)

Stranj that he does not just mention it by name or go into mor detail, or eevn sujest mor litratur on it.

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