What does secular humanism (or, as we say in the UK, humanism) involve? In Humanism: A Very Short Introduction (OUP 2011) I suggest that most of those who sign up to secular humanism sign up to following:

1. Secular humanists place particular emphasis on the role of science and reason.

2. Humanists are atheists. They do not sign up to belief in a god or gods.

3. Humanists suppose that this is very probably the only life we have.

4. Humanists usually believe in the existence and importance of moral value.

5. Humanists emphasize our individual moral autonomy and responsibility.

6. Humanists are secularists in the sense that they favour an open, democratic society and believe the State should take neutral stance on religion.

7. Humanists believe that we can enjoy significant, meaningful lives even if there is no is a God, and whether or not we happen to be religious.

Now some readers may be thinking, ‘But hang on, you haven’t mentioned naturalism. Surely secular humanists also sign up to naturalism, right? They reject belief in the supernatural. So why no mention of naturalism here?

Well, let me say at the outset that of course a good many secular humanists do sign up to naturalism. And perhaps rightly so. Secular humanists should be free to embrace and argue for naturalism if they wish. And perhaps a good case can indeed be made for naturalism. What I am suggesting is that it is a strategic mistake to define secular humanism so that it entails naturalism. So far as secular humanism is concerned, signing up to naturalism should be an option, not a requirement.

Why do I suggest it’s a strategic mistake to define secular humanism so that it entails naturalism? For two main reasons:

1. because it unnecessarily excludes many from the secular humanist club who could and should be invited in.

2. because it creates an unnecessary hostage to fortune.

To illustrate the first point first, note that naturalism is pretty controversial even amongst atheists. Take the professional philosophical community. The 2009 Philpapers survey of the opinions of professional philosophers and graduate students revealed that less than 15% of professional philosophers and graduate students are theists. Yet only a little less than half of them sign up to naturalism. That leaves around 35% who are neither theists nor naturalists. Why bar them all entry to the secular humanist club, particularly when many of them will be fully in agreement with points 1-7 above (which are, it seems to me, the points that really matter)?

Why are some philosophers sceptical about naturalism? Not because they have much time for belief in gods, angels, fairies, goblins psychic powers, telekenesis and other spooky phenomena about which naturalists are, of course, rightly sceptical. Most atheist non-naturalist philosophers are no less sceptical.

Philosophical doubts about naturalism tend to spring, first, from concerns about whether naturalism is a well-defined concept. What is naturalism (or metaphysical naturalism, to be precise)? A sceptic’s usual first port of call is to say that naturalism consists in the rejection of belief in the supernatural. But what is the supernatural? Why, it’s that which isn’t natural, of course! But now notice that these explanations of naturalism and supernaturalism are, as they stand, entirely circular and uninformative. So far, no significant meaning has been attached to either term. It’s harder to define ‘naturalism’ than you might think.

Second, some reject naturalism because they suspect that, for example, mathematical Platonism might be true. Many mathematicians believe mathematics describes a non-natural mathematical realm. They suppose that ‘2 + 2 = 4’ is made true by how things stand in this external, non-natural, mathematical reality. If such a mathematical reality exists, then naturalism is false (though it strikes me as odd to describe this reality as ‘supernatural’ given that term’s ‘spooky’ connotations).

It is also philosophically controversial whether minds, or moral value, can be accommodated with the natural realm. There are, notoriously, all sorts of philosophical objections to naturalism. I am not entirely confident all these objections can be dealt with. So, though I personally lean towards naturalism, I’m by no means committed to it. I’m one of the 35% of professional philosophers that’s neither theist nor naturalist. So if ‘secular humanism’ is defined so that signing up to naturalism is a requirement, rather than just an option, then I’m excluded from the club. And so are very many other sensible folk who nevertheless tick all seven boxes outlined above. So far as the secular humanist movement is concerned, excluding such individuals is, it seems to me, a strategic mistake.

I said that a second reason it’s unwise to make secular humanism entail naturalism is that it provides an unnecessary hostage to fortune. Why so? Well, if secular humanism entails naturalism, all a theist has to do to refute secular humanism is refute naturalism. And, as I say, there are all sorts of philosophical arguments against naturalism ready to hand. If we define secular humanism so that it entails naturalism, critics will rub their hands together with glee knowing we have just provided them with a cupboard full of stock philosophical objections. Whether or not any of these objections are good, many are troublesome enough to get secular humanists needlessly bogged down in a philosophical quagmire.

In response to these sorts of objection (‘But how does mind, or moral value, or mathematical truth, fit into your naturalistic world view?’), those who sign up to secular humanism as outlined above should not attempt to defend naturalism but should instead simply shrug and say, ‘So what? Even if your objections successfully established that naturalism is false, that would leave both my atheism and my secular humanism entirely unscathed. What are your arguments against atheism and secular humanism?’ The moral is: don’t get bogged in unnecessary battles that you might lose, and that you certainly don’t need to win, in order successfully to defend atheism and secular humanism.