Caspar W. Weinberger is very like his boss, President Reagan. He displays the same indifference to reality, dismisses or ignores his own mistakes and, when the occasion calls for it, repeats them.

Another secretary of defense might be defensive in the light of the waste fraud and abuse revealed within his domain. Not Weinberger. He stoutly maintains that his own alert watchdogs unearthed the scandals, although this has rarely been the case -- and the record shows that Pentagonians who bark about highway robbery among contractors or defects in weapons systems are regularly exiled to Alaska or reassigned to desks where they can't see the books or the duds.

Like Reagan, Weinberger sticks to a few simple themes: The Soviets are ahead; they are cheating. After five years and a trillion dollars in Pentagon spending, we are dangerously behind.

He and his principal differ only in tactics. While the president is almost always a model of "gee-whiz" amiability, Weinberger is like a defense attorney in a criminal case. He fights for every inch of ground. He treats his adversaries to a bristling, edgy hostility and when challenged, suggests witheringly that his questioner is someone who is willing to put the country at risk of Soviet domination.

Members of Congress rail against Weinberger's obduracy. But predictions that Reagan will have to divest himself of Weinberger, as is often rumored, are hollow and false. When Reagan listens to Weinberger, he hears himself talking.

The secretary of defense is anathema to arms controllers; he shares the president's aversion to accords -- and is, paradoxically, the last best hope of those who hope to avoid a hot war in the here and now.

He is the one member of the Cabinet who has demonstrably learned the lesson of Vietnam. The Joint Chiefs of Staff have told him that we must never again start a war without popular support. While some hotheads want to invade Nicaragua and "get it over with," Weinberger demurs. He was opposed to the dispatch of the Marines to Lebanon. He collects weapons the way Imelda Marcos acquired shoes, but he does not want to use them. Weinberger likes to look at his arsenal. But he hates to hear shots fired in anger.

He is probably the best secretary of defense that could be hoped for in a Reagan administration.

He is currently engaged in a new and epic struggle against the facts. The whole government is facing drastic cuts under the Gramm-Rudman-Hollings balanced budget law. He is asking for a 12 percent increase. Weinberger will not designate a single gun, bullet or body to be given up.

He says that the solution is to adopt the president's budget. Several members of the House Appropriations defense subcommittee, where Weinberger testified yesterday, pointed out that the president's budget got just 12 votes on the House floor.

But he briskly says he has authority only to talk about the increases.

Rep. John P. Murtha (D-Pa.), a hawk, told him that he should be out peddling a tax increase to cover the rising costs.

Weinberger paid scant respect to that heresy: "I haven't heard anybody say it [a new levy] would go to defense. We don't have to have Gramm-Rudman. There is simply no way we can absorb cuts of this kind."

One way to reduce the Pentagon purse, of course, would be arms control. Weinberger will have none of it.

Rep. Norman D. Dicks (D-Wash.) brought up the possibility and was brusquely put down by Weinberger. He turned to Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff. Did he know if the president would abide by the SALT II treaty?

The admiral, a heavy-faced man, said the chiefs had given the president advice, but declined to say what it was.

"It is pretty clear that the Soviets are violating SALT II. I am convinced of it," he said as Weinberger beamed at him.

Hadn't the Soviets dismantled 14 Yankee Class submarines? Dicks asked.

"I don't know the exact number," said the admiral negligently, his tone indicating that the Pentagon has no time for such fripperies.

Rep. Les AuCoin (D-Ore.), an inveterate arms controller and author of the one arms control advance in the last five years, an amendment banning antisatellite weapons testing, asked if "Star Wars" was really necessary.

"It is extremely important," Weinberger said. "The Soviets have been working on it [a space-based defense system] for 17 years."

And he said, echoing one of Reagan's biggest whoppers, "We don't want it for unilateral advantage. When we get it we will share it."

Star Wars will cost another trillion dollars. But with Weinberger, as with the president, on defense there is never enough.