The hon. Gentleman is entitled to his view on that, and so, indeed, is Dr. Thompson, whom he has often quoted. Dr. Thompson has his views on the matter, but it is worth noting that after he expressed his views, Edinburgh university made it clear that they were not necessarily the views of the university, but were Dr. Thompson's personal views. I have no objection to that. If the hon. Gentleman listens to my argument as a whole, he will get an answer to the whole question of the rationale of why we have to take account of the reality of what is happening, and not the situation as we might wish it to be.

The essential question is whether defensive systems have a role to play in maintaining deterrence and ensuring strategic stability, or whether we should continue to deter strategic nuclear attack solely by the threat of retaliation. This issue is, of course, not new. Both the USA and the USSR worked on defensive systems in the late 1960s. It became clear that the technology then available would not provide cost-effective defences against the weight of attack which the super powers could pose against each other. The right hon. Gentleman touched on that point. Both sides accepted a different twin-pronged approach—numerical constraints on offensive sytems, coupled with strict limits on defensive deployments. That resulted in the signature of the 1972 SALT I and ABM treaties. Each side relied for its security against strategic nuclear attack on the threat of retaliation from its own nuclear forces, which were themselves virtually immune from preemptive attack.

Those treaties have provided the strategic framework under which we have lived ever since. They make up the concept known as mutual assured destruction or, as my right hon. and learned Friend the Secretary of State for Foreign and Commonwealth Affairs more accurately and realistically described it, mutual assured deterrence. The Government attach great importance to the integrity of the SALT and ABM treaties, which marked a real achievement in an agreed approach to ensuring a stable and peaceful world.

It follows that we believe that their terms should be strictly complied with by all the parties to them. That is why we very much welcomed the US commitment to pursue its SDI research programme in accordance with a strict interpretation of the ABM treaty and not to undercut the unratified SALT II agreement as long as the Soviet Union exercised equal restraint.

It is, of course, entirely to be expected—indeed., it is all too typical—that the focus of the Opposition motion should be on the United States strategic defence initiative. We shall hear much today—we have already from the right hon. Gentleman —of how it is the United States that is upsetting the approach agreed in the 1970s. We shall, perhaps, be left with the impression from some speakers that the Soviet Union, on the other hand, has respected that approach and wants nothing to do with strategic defences. The reality is very different.

It is demonstrably not the case that the Soviet Union has eschewed the possibility of strategic defences. It was the Soviet Union that chose to make full use of the provision for the limited deployment of such systems permitted under the ABM treaty by deploying an ABM system around Moscow — a system which it is currently modernising and expanding to the extent allowed by that treaty. In the days when those on the Opposition Benches were still being really serious about defence, it was precisely that Soviet deployment that led them — responsibly and properly — to proceed with the Chevaline programme for our strategic nuclear deterrent. The right hon. Gentleman's famous memory was not quite correct today. Although it was a Conservative Government who decided to go ahead with Chevaline, it was a Labour Government who made the arrangements and allocated funding. He cannot get away from that.

Perhaps this limited ABM deployment might at first have been explained away as a hangover from the earlier Soviet R and D effort in the 1960s, but it is now being upgraded. Above all, we have to address the long-standing Soviet research programmes into the range of technologies relevant to ballistic missile defence — lasers, particle beam and radio frequency weapons, kinetic energy weapons, surveillance and target detection and so on.

My right hon. Friend the Minister of State for the Armed Forces placed in the Library on 26 November 1985 a paper on that Soviet programme, which I commend to the House and to hon. Members who have not read it. The facts that it sets out are, I believe, not in dispute. The key point is that this is not a new Soviet programme; it is not a response to the SDI— far from it, it long pre-dates it—it is not something peripheral to the Soviet effort in defence research; it is a key component of it.