AN AXIOM OF long standing is that "nothing is more certain than death and taxes." At the same time, whereas it is generally held that there is life after death, it has also been held that once an estate was settled, there would be no more taxes after death.

So taxpayers who looked forward or upward to the relief from taxes as one of the joys of the Hereafter must have been deeply disillusioned by a recent announcement from the Treasury Department.

In anticipation of a nuclear attack and the destruction, confusion, and near chaos that might follow, a senior Treasury official has prepared a plan "for collecting taxes even under those difficult conditions." Even annihilation does not bring escape from the IRS.

The plan is called "A Design of an Emergency Tax System." The "design's" first concern is to perpetuate the Individual Income Tax system by securing the records on the basis of which taxes are determined. Taxes owed by those who are annihilated will be assessed as best they can be, the report states.

It would be a convenience to the IRS if the nuclear attack could be coordinated with the April 15 deadline (no pun intended) for income tax payments. If that date is not possible, perhaps the powers that be could manage it on or soon after one of the days on which quarterly payments of estimated taxes are scheduled.

The Treasury "design" does not make public its plans for the safety and survival of IRS agents. Possibly the agents may be lost, but in the short run -- for five or six months after the attack -- the computers, in some safe place, will continue the essential work of the IRS.

The "design" notes that nuclear war might be so disastrous that in addition to the destruction of millions of people, major industrial installations, and major population centers, it might even destroy the tax system. This, the IRS seems to believe, would be the most serious consequence of nuclear war. There are some experts in post-nuclear war survival who hold that along with cockroaches, the income tax system is likely to be among the few survivors of such war.

Taking no chances, however, the author of The "design" proposes a stand-by tax program in the form of a general sales tax to be applied at the point of purchase.

Such a tax, the author holds, would have two advantages -- it would encourage savings and "aid in rebuilding the capital stock" of the country.

The Treasury expert has even set the percentage level for the tax: 20 percent. He says this should do it. That does not quite square with the Mutual Assured Destruction concept, developed by Robert McNamara as secretary of defense, which held that deterrence would occur at the prospect of the loss of 20 percent of the population and 50 percent of industrial capacity.

The "design" does not make clear who will collect the tax or whether it will be applied to the costs of mortician services, although in the immediate post-bombing period such goods and services would make up a major part of the gross national product.

The danger with a plan of this kind, a contingency plan, is not in its being there in anticipation of the emergency for which it was drafted. It is probably as a good a plan as any that could be devised for conditions which no one can anticipate.

The danger lies in the fact that Treasury and the IRS may become attached to the plan to the point that without the nuclear war for which it was prepared, they may come to believe that the plan offers a better tax system than the one currently in place. They might offer it as a substitute, with the sustaining argument that apart from the merits of their tax program, it would be a good idea to have the emergency program established in advance of the emergency.

A contingency plan can be a destabilizing force, as those who conceived it may become more and more attached to it, and eager to test it, with or without an emergency.

The attempted takeover of a military government in Greece a few years ago by another military group is a recent example. The attempt was based on a contingency plan to take over the government if it were communist controlled -- which this one was not.

The lesson is to beware of bureaucrats (or generals) bearing contingency plans. Possibly, under bureaucracy, you not only cannot take it with you, but you cannot even leave it behind.