"If God is not, everything is permitted." Or so they say. Except they're wrong. Dangerously so.

This dictum - that without some absolute divine authority, then morality is at best arbitrary, at worst, annihilated - is unsheathed and bandied about all-too-often these days.

Recently, it's reared its seditious head in response to the trial of an ethics-based complement to scripture in NSW. The church has pulled out all the stops to block the ethics class, and one of the reasons posed is that ethics without God is hollow, that teaching secular ethics is like teaching English without books, maths without numbers, science without observation.

But the notion that God is required in order for morality to have any real clout is demonstrably false. In fact, if you want a comprehensive, robust and flexible ethics that can address the problems we face today, then you need to explicitly look for a morality without God.

This is because the subject matter of morality is very much grounded in the real world: morality deals with real people, real issues and has to navigate real conflicts. And the real world is a complicated place where not everything is as it seems. One of our best tools for understanding the real world is the humble question "why." But often you have to ask "why" more than once to get to the answer.

Why is the bus late? Why did the driver not leave on time? Why wasn't his bus ready for him at the depot? Why is the NSW government still in power? And so on.

To get to your answer, you need to be able to ask "why" as many times as necessary - at least until you exhaust all possible evidence (as in the scientific process) or all possible reason (as in the philosophical process).

But religion stifles this process, with dangerous consequences. This is because religion, by its very nature (no pun intended), is grounded in the supernatural. Its teachings hinge on belief in beings, forces or realms that cannot be seen, felt or known without resorting to faith.

This means that when trying to understand the world from a religious perspective, you can only ask "why" so many times before you hit the brick wall of the supernatural. There's a point where the answer to your last "why" is simply "because God/the Bible/the Space Fairies said so". And if you're not happy with that answer, tough. You just lack faith.

Unfortunately, when it comes to constructing a robust and reliable morality, the supernaturalist approach is horribly prone to error. One belief held dogmatically on supernatural grounds can yield moral outcomes that end up causing untold harm, such as the Catholic prohibition on contraception, for but one example.

That's not to say a secular approach isn't also prone to error. But, the big difference - the difference that really counts - is that the secular approach is always open to scrutiny. It always allows for others to ask "why" about any of its moral prescriptions. And, as such, it is open to revision in light of new evidence or new arguments, and it's more easily able to correct its errors.

The suggestion that we need some supernatural authority to compel us to obey the moral law - well, that's also bunk.

This is because morality - whether it's justified by reason, nature or the divine - is, and always has been, believed, doubted and argued by everyone. Even the most dogmatic religion has experts - anointed or appointed - who debate the interpretation of the scriptures. And practitioners of even the most dogmatic religions are known to stray from the path, only to be guided back, by carrot or stick, by their peers.

The same is true of secular morality. The reason we behave morally is partly psychological, partly ideological and partly through desire for praise and fear of punishment. Whether the ideology is backed up by some supernatural power makes no difference in practice to whether the morality is persuasive or not; non-supernatural forces can be terribly persuasive, just ask Fergie.

Ultimately, the argument that 'without God, anything goes' is just plain false. There might be other reasons to question secular morality, or to support religion, but let it not be that morality requires God. It doesn't. Morality will only be stronger and better able to deal with the pressing problems that we all face if it is free to question the world and itself. That kind of ethics ought be taught in school.

To not do so would be, well, immoral.

Tim Dean is a science journalist and philosophy PhD student.