John Sulston is one of the smartest men I know – well, he ought to be, as a Nobel prize winner – and last week I got him talking about religion in front of an audience for the Westminster faith interviews.

One of the things that came up in this, as so often before, was the definition of "religion". Sulston was brought up as a low church Anglican, and still feels that religion must involve God and a belief in the supernatural, and that ritual is secondary to theology.

I came up with my usual counter to this – that there are atheistic religions; that there was ritual long before there could be theology and that we ought to take scientists – even social scientists – more seriously than dictionaries. This last point because Sulston had gone to the trouble of looking up and printing out one of the OED definitions of religion, which he felt proved his point.

"Belief in or acknowledgement of some superhuman power or powers (esp a god or gods) which is typically manifested in obedience, reverence, and worship; such a belief as part of a system defining a code of living, esp as a means of achieving spiritual or material improvement."

I can see that it must be frustrating, if you have such a definition in front of you to get some slippery Durkheimian answer about religion being actually the way that society understands and defines itself. You might, if pressed, agree that Americans treat their constitution as a sacred scripture, of universal application to the world. But it doesn't seem properly supernatural.

This is probably an argument that is impossible to resolve. But every serious thinker about religion has ended up with a definition as baggy as Durkheim's. There are just too many modes of belief and behaviour that can function as "religious" for this to be a simple category. And if the dictionary says different, then the dictionary is wrong.

The same holds true, of course, for things like evolution: if I want to know what evolution means, I ask biologists, not dictionaries. The meaning that scientists use may not be more correct than the popular one – how would you measure that? – but it is going to be much more useful for investigations of the subject. So, I am quite happy to say that science could function as a religion, in some modes and in some societies, while at the same time functioning as science. And it ought to be perfectly possible to distinguish between the two uses. Here's how.

Let's imagine that some future alien archaeologist, poking around in the remains of the Earth, comes across the pyramids and also the great circular tunnel, stripped of its machinery, which once contained the Large Hadron Collider outside Geneva. Unless alien archaeology really is unimaginably alien, both of these giant structures, whose purpose is not immediately obvious, will at once be labelled "ritual structures". They were both feats of engineering, which required huge co-ordinated efforts, inspired by a belief in some non-material, transcendent good. In both cases, the people served a priesthood whose knowledge they had to take on faith. So, is there any test that lets us say that the one enterprise is "scientific", and the other "religious"? It seems to me that there is one.

If you were to work out the purposes of the two structures and try to build working models, the LHC tunnel would produce the same results in the alien future as it does now. The same experiments would produce the same evidence of the Higgs boson. The pyramids could not work like that. We could rebuild a pyramid, stock it with grave goods, and entomb some deserving, eviscerated and mummified world leader inside it. But at the end of these operations we still wouldn't know what Egyptians believed, or how. We certainly wouldn't know that the world leader had passed into the afterlife. If we did – if ritual actions had predictable, if inexplicable, results – that wouldn't be religion, but magic.

There's a very wonderful poem about this inability to understand the religions of the past by the Swedish Nobel laureate Harry Martinson, translated here and in the recent collection Chickweed Wintergreen.

Scientific and religious explanations come together in an odd way at Stonehenge and similar monuments. They can be interpreted as megalithic calendars, or devices for astronomical prediction, as well as ritual burying grounds – and the reason we can reconstruct them as gigantic observatories is precisely that we can calculate today exactly what would have emerged from calculations done 4,000 years ago.

Yet to call Stonehenge a purely scientific enterprise is clearly wrong. When you consider the immense labour and complex social organisation required to put all those stones in place, you could be inspired to ask "where would the sun have risen at midsummer 3235 BC". But surely the much more interesting question is why this question should have been thought so important in that culture.

That seems to me a question that only historians and sociologists of religion can answer. What's more, although the scientific question and its answer are independent of any particular cultural and religious matrix, they can't be independent of all of them.

To come back to Sulston – anyone who had sequenced the same material as he did would have come up with very similar results. That's the scientific question and it's the one that interested him. But the money and the resources that made it all possible were not raised by an appeal to intellectual curiosity and probably could never have been. They were raised partly in the expectation of profit, and partly by politicians using a largely religious rhetoric about "The book of life" which all the scientists involved could have explained was nonsense and which would certainly be impossible for an alien archaeologist to reconstruct. Yet the funds would never have been voted without it. So: is the Genome Centre a scientific factory or a ritual centre? It's both, and that's why the dictionary is wrong.