This trenchant remark is attributed to Dwight Morrow, the American ambassador to Mexico during the administration of Calvin Coolidge, Ronald Reagan's favorite President:
''The trouble is, we judge ourselves by our motives and others by their actions.''
One is reminded of it by some remarks of Reagan himself. Arguing for the MX missile before the American Legion, the President denied that it would pose a first-strike threat to the Soviet Union. This idea, he says, ''runs counter to the whole history of America. Our country has never started a war, and we have never sought, nor will we ever develop, a strategic first-strike capability.''
To somebody with Reagan's mind-set even to suggest that we would launch a first-strike is preposterous. Never mind that some historians might quarrel with the bit about never starting a war - the Spanish-American War in 1898, for example. The United States is a peace-loving country. Its leaders - not only Reagan - have been bragging about this for years. The purity of our motives makes us worthy of trust.
But consider how things look when seen from Moscow. Reagan is probably correct that the MX will not in fact give the US a first-strike capability. Soviet missiles are so numerous and so widely dispersed that destroying all or even a major fraction of them at one time is a practical impossibility, no matter what theoretical models may be built by a computer. The Soviets doubtless know this. But they also know that, first strike or not, the MX would add significantly to American nuclear firepower. And they are less interested in American intentions than in American capabilities.
This, of course, is precisely the logic of the Reagan Pentagon - and of generations of military planners, for that matter. It produces the argument that we have to plan our military forces not on the basis of what a potential enemy is likely to do, but on the basis of what he could do. If unrestrained by considerations of other national priorities or of resource allocation, this is what leads to the endless accumulation of ever more complicated and destructive weapons. When two opposing superpowers proceed on this premise, it makes the world a scary place.
When the President argued that regardless of intentions the MX would not in fact give the US a first-strike capability, he was denying one of his own principal arguments for building the MX. That argument is that the increased accuracy of Soviet missiles has made it possible to wipe out all the US Minuteman silos at once. This would preclude retaliation against a Soviet attack and would give the Soviets a first-strike capability.
Therefore, the Reagan administration has argued, the US has to build the MX to regain the deterrence it has presumably lost. This argument has always been faulty on two grounds. First, it assumes a most improbable technical capability by the Soviets. Second, it completely overlooks the existence of US airborne and seaborne nuclear forces. What are we supposed to think these forces, especially the submarines, will be doing while the continental US is being incinerated? And if the European public will hold still, in a few months we will have Pershing missiles in NATO. Would it not occur to somebody to fire them?
For all their callous behavior, the Soviets are not complete idiots. We can assume that these points have also occurred to their military planners. They are no doubt aware that they do not have a first-strike capability any more than we do. Nor is either side likely to acquire it.
''There is no way that the MX, even with the remaining Minuteman force, could knock out the entire Soviet ICBM force,'' Reagan told the American Legion. Why, then, does he think the Soviets could knock out the Minuteman even without the MX? He cannot have the argument both ways.
Assuming a reasonable degree of sanity in Washington and Moscow, the danger of the world ending not with a whimper but a bang does not come from an attempted first strike. It is more likely to come from the uncontrolled escalation of a superpower confrontation which starts small and gets big.
Arms control agreements are an important step in putting an upper limit on how big a confrontation could get. With weapons already in American and Soviet inventories, it could get big enough to blow up the world; but we've got to start somewhere. Arms control agreements by themselves, however, do not keep a confrontation from starting small. That was - and remains - the necessity for the unfortunately discredited Nixon-Kissinger policy of detente.
Before the United Nations last week, Reagan sounded as though he really wants strategic arms reductions. But neither there nor elsewhere has he sounded as though he even understands the importance of reducing tensions.