President Reagan answered a question never asked him last week, providing one of the few jarring notes in what was otherwise the smoothest news conference of his presidency.

Discussing a possible summit meeting with Soviet President Konstantin U. Chernenko, Reagan launched into rhapsodic praise for the unity of the U.S. team that accompanied Secretary of State George P. Shultz to Geneva for talks that produced agreement on new arms-control negotiations. Reagan said the U.S. delegation displayed "complete unanimity" and added that "there has been no infighting, as some have suggested, about what we are going to talk about over there."

The comment suggests that Reagan is so far distanced from arms control decisions within his administration that he must be viewing them from another planet. Only a week before, in an interview with Fred Hiatt of The Washington Post, arms control nemesis Richard N. Perle, assistant defense secretary for national security policy, said of the State Department view: "They consider it their mission to achieve agreements rather than to protect the interests and programs and military balance that is the substance of these talks."

Perle has the virtue of being straightforward and knowledgeable. But he is also a devoted foe of arms control at a time when the president is proclaiming that "a more stable peace is achievable" through the new negotiations.

Reagan's persistent refusal to acknowledge the unresolved conflicts fuels speculation, however unjustified, that he is more devoted to the politics of peace than to an actual agreement with the Soviets.

Increasingly, as second-term battle lines are drawn, these conflicts have become touchstones for conservative and moderate forces within the administration. The conservatives, using Reagan phrases of the past, argue that arms agreements are worthless with a nation that doesn't respect treaties or human rights. For mass consumption, their reference point is apt to be the murderous conduct of Soviet troops in Afghanistan.

The conservatives' bottom-line argument is that arms control agreements lull Americans into thinking that the Soviets have changed their ways and make it difficult to win public support for military spending.

But the United States has conducted its biggest defense buildup since World War II while observing the SALT I treaty and living up to the unratified SALT II agreement. Reagan has proved more effective than any other postwar president in rallying U.S. support for a strong defense. With Afghanistan as a reminder, it is unlikely that any arms control agreement would prompt Americans to confuse any Soviet leader with Rebecca of Sunnybrook Farm.

The moderates, and Democratic critics of the administration, have problems with the U.S. position. One is Reagan's commitment to the Strategic Defense Initiative, usually known as "Star Wars." The Soviets, ignoring their investment in missile defense, see the SDI as a barrier to any agreement, and some within the Reagan administration privately view it as a bargaining chip to be traded.

But the SDI is in no condition to be traded. It fits neither its opponents' apocalyptic descriptions nor Reagan's grandiose hope of finding a means of "rendering ballistic missiles impotent and obsolete." The defensive research that so many Democrats denounce was being conducted under President Jimmy Carter in separate programs on which $1.8 billion would have been spent in fiscal 1986.

Reagan collected these programs under one roof, gave them a fancy name and asked Congress to double the funding in 1986. No one has shown that this gussied-up initiative is producing more than the programs it replaced.

On "Star Wars," Reagan's communicative skills may have done him a disservice. He succeeded so well in conveying the impression that a defensive shield was achievable that his critics have reacted to the vision rather than the reality. Reagan recognized this at his news conference, where he played down the SDI as a research program without definable outcome.

What Reagan didn't recognize, and should, is that the U.S. delegation's euphoria at Geneva conceals a wide range of differences on Star Wars and other issues that are sure to surface when arms-control negotiations begin. It is unlikely that the president can resolve the differences as long as he refuses to recognize that they exist.

Reaganism of the Week: Asked in an interview with The Dallas Morning News last Tuesday whether he is ready to endorse the Treasury's tax-reform proposal, Reagan said he had a copy of the plan but hadn't read it. "And while I've read a summary, I haven't been able to do that because my mind is too filled with the budget," he said.