Half a millennium ago Machiavelli insisted on the importance of law if mankind was to live in peace. The source of law, he thought, should be an all-powerful authority; the Prince. But he sensed a weakness in his prescription, since he went on to say, appositely: “The human tragedy is that circumstances change, but man does not”. How is law to adapt? The only answer on offer is that the law-makers consult with their public, legitimising change.

The greatest single “change in circumstance” demanding new laws, in the centuries since Machiavelli, has been the rise of science. Science is itself a body of laws; laws of nature, and also laws governing scientific conduct. Both derive their force from consultation in the community.

It is sometimes overlooked that the power of science comes from debate — that science is grounded in democracy. There is a caricature of science as being comprised of a catalogue of facts. If this were the case there would be no need for the scientific meetings, since we would not need to debate. Facts would be transmitted online to a central location where they would trigger a round of applause.

But that is not our experience of scientific meetings. Rather than using them to state facts, we advance propositions. As with evidence in court, these are tested in cross-examination before a jury of our peers.

Truths established in this fashion can subsequently be overthrown by a higher court. Indeed, on examination the new laws of science often turn out to be approximations. This serves as a reminder that the Creator did not originate them; human beings did. Human constructs though they are, they give evidence of their power by opening a Pandora’s box of possibilities.

Though today’s symposium meets in alarm because of the impact of science, science remains our greatest hope. This is not just because science marshals the power of reason, but because it does so in a civilised way.

We must hope that the same civilised approach can be employed to persuade a wider public of new truths. It is, in fact, our only hope. We cannot conduct ourselves on a crowded planet as we please; the economy will fail (as, to a degree, it has), the environment will fail (as it has begun to do) and the victims of both failures may, in desperation, take up arms. I ascribe to this last possibility the revival of concern over the spread of nuclear weapons.

There is serous talk today of a global bargain in which the nuclear “haves” renounce their weapons in order to delegitimise their possession. Being numerate, scientists should play a part in that debate, insisting on the difference between zero and a thousand. No one here, faced with the nightmares of weapons proliferation and climate change, doubts the imperative to regulate international behaviour. And no one, surely, thinks that we can do that simply by fiat. If we have learnt anything in recent times, it is the limits of power as an instrument of persuasion.

We will need, instead, to imitate the methods that science uses to legitimise its propositions. The task before us will be harder, since the community is wider. At times it will seem impossible — but we have experience of that, too, in science. Our fundamental beliefs — about matter, motion, life and the cosmos — have had to be revolutionised several times. Our community has emerged intact and strengthened.

At one time it would have been thought mistaken to suggest that scientists meddle in politics. Today it would be shameful to deny that they have this responsibility.

At the dawn of the atomic age, scientists began to emerge as a political force. In the early summer of 1945, in advance of the testing of the first atomic bomb, a group under the chairmanship of James Franck (a Nobel laureate) sent a “position paper” to the US Secretary of War, Henry Stimson. “In the past”, they said, “science has been able to provide new methods of protection against new weapons. Given the devastating power of nuclear weapons,” they continued, “this would no longer be the case in the future.”

These atomic scientists had correctly identified the central characteristic of the new age: the age of vulnerability. They went on in the same document to call for nothing less than a political re-organisation of the world, so that international agreements could supplant national defences. This came to be known as “collective security”.

The small group formulating that statement in Chicago may have felt no one was listening. If so, they were wrong, for there followed an avalanche of agreements bringing international law to centre stage.

This great transformation, in which scientists played a part, represents the world’s best hope for the future: the establishment of agreed laws which, by degrees, tie the hands of states and empower the international community.

Central was the adoption in that same year, 1945, of the UN Charter subjecting states to rules governing the initiation of war. The Charter took the logical further step of making it the responsibility of the UN to act on behalf of world peace.

This it does sporadically through bodies devoted to international justice, trade, banking, development, disarmament, peace-keeping, environment and human rights. Attempts are, of course, made to subvert this new order, but the international framework survives because the majority abhor a return to the jungle.

We seek visible and continuing progress to a new order, not Utopia. We have had enough Utopias, each more awful than the last. All have been based on a false notion of the incontrovertible nature of scientific proof; a misunderstanding that provided a perfect ground for tyranny. In fact, science persuades, it does not compel.

A paradox lies at the base of the gleaming edifice of science. Science’s greatest gift to civilisation is its acknowledgement of fallibility. Those of us who do science know full well that there are no final answers. There is only a creeping progress out of darkness into light.

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