Humanists are often assumed invariably to be, and certainly often self-identify as, naturalists. Indeed, humanism is regularly defined in such a way that signing up to naturalism is a requirement.
I am sympathetic to naturalism. However, like many philosophers, I am not fully committed. The PhilPapers survey of professional philosophers and graduate students revealed that while less than 15% of them sign up to any sort of theism, only around half are committed to naturalism. Why so? Are such philosophers undecided about, if not God, then other supernatural hocus pocus in the form of ghosts, angels, demons, spirit beings, and so on? Are they agnostic about woo?
Of course not. I am highly skeptical about such supernatural claims, and perhaps that’s enough to qualify me as a naturalist. However, ‘naturalism’ is a slippery term, and one reason some philosophers aren’t happy about applying the term to themselves is that it’s hard to define. We might start off with this: naturalism = rejection of the supernatural. But if we then define ‘supernatural’ as that which naturalists reject, our definition remains uninformative. I am not suggesting there aren’t robust definitions available. I am just pointing out that defining the term is not as straightforward as we might initially assume.
Another worry about not just giving humanists the option of being naturalists, but requiring it of them (and kicking them out of the humanist club if they fail to sign up) is that there are all sorts of atheist positions that are in tension with naturalism. Mathematicians, for example, are very often mathematical Platonists. They believe mathematics describes, and mathematical propositions are made true by, how things stand in some sort of non-natural mathematical realm. Can such atheist mathematicians be humanists? I’d say so. Why exclude them from the humanist club?
Finally, a commitment to naturalism provides an unnecessary hostage to fortune when we’re debating religious folk. The religious often assume that atheism and humanism both involve naturalism. If we define humanism so that naturalism is indeed a requirement, then, in order to refute humanism the religious need only refute naturalism. The religious come armed with a vast array of arguments against naturalism all of which we will then have successfully to refute. Watch their faces fall when, after they have unleashed a barrage of arguments against naturalism, you point out that you are defending humanism, not naturalism.
A successful argument for mathematical Platonism would surely be irrelevant so far as whether humanism is true is concerned. So why choose to define humanism in such a way that such an argument would actually successfully refute humanism?
Having said that, as long as naturalism is understood as a commitment to no spooky stuff (ghosts, fairies, spirit beings), then sign me up. And as I suppose that is how many humanists understand them term, then perhaps it’s even fine to define humanism as a requiring not just a commitment to no gods, but a commitment to naturalism. I can understand why many humanists would be uncomfortable about abandoning that the requirement of naturalism, particularly if they suppose we would thereby giving a green light to New Age supernatural baloney, say (heaven forbid!)
What is most important here is that we get clear about exactly what we mean by ‘naturalism’, and that we use the term with care, so that we humanists are neither (i) misunderstood as allowing in hocus pocus and woo by rejecting it, nor (ii) misunderstood as kicking e.g. the mathematical Platonists out by accepting it.