"Thank God he's not black."

The words spilled out almost reflexively last Monday, all over town -- perhaps all over America, particularly black America.

Maybe later we would say that we were glad no one had been killed in the hail of bullets that felled four men, including the president. But first: Thank God that the man accused in the assassination attempt wasn't black.

And you find yourself thinking: What if he had been? So far, it is true, none of the assassinations and attempted assassinations of presidents have been the work of black people. Indeed, nearly all the assaults on national political figures have been at the hands of whites, the apparently conspiratorial slaying of Malcolm X and the knife attack on Martin Luther King being the most notable exceptions.

But it is also true that most of the attackers have been crazy people -- assassins or would-be assassins without clear motive or grievance. And mostly we talk, in the aftermath of the assaults, of how we might understand these sickos and how to prevent recurrences of their pointless violence. We don't suppose that the violence of a specific nut -- a Squeaky Fromme or an Arthur Bremer -- has any particular long-term implications for white Americans.

Why do we assume almost automatically that the violence of a crazy black man should bring down the nation's wrath on all blacks? I'm not suggesting that the assumption is unfounded, only that it says some very worrisome things about the state of race relations in America.

Nor is it a one-way street. Millions of Americans, including a significant number of black Americans, are offering earnest prayers that the killer(s) of the children in Atlanta will turn out to be black. The fear, perhaps equally well-founded, is that black America will not be able to look at a white murderer of black children as merely an isolated psychopath -- any more than white America could view a black assassin of a white political leader as just another nut. Celebrated murders that cross color lines seem almost automatically to evoke racial reactions.

A white reader of The Washington Post has suggested how easy it might be to turn Monday's assassination attempt into an ethnic conspiracy. She pictures a ruddy-faced resident of South Boston playing to the assembled crowd:

"Fellow Irishmen!" she has him shouting. "I am here today to ask you just the one question: Is there a man among you who believes that one single crazy fellow pulled a trigger six times in Washington and hit four men -- four men named Reagan, Brady, McCarthy and Delahanty -- and hit them by accident?

"And is there a man among you who believes that just the one crazy fellow, acting alone, gunned down John Fitzgerald Kennedy? And believes the same about Robert Francis Kennedy?"

The woman, who thinks her own antecedents may be Irish, doesn't believe there is any such anti-Irish conspiracy, of course. But you do see her point.

If Monday's attacker had been a fired CETA worker, or an outraged liberal, or the head of a family whose food stamp allotment had reduced his children to hunger, it might be fair to wonder whether his outburst was the first portent of violent reaction to the coldhearted policies of the Reagan administration.

Obviously, it isn't logical to make such extrapolations from the actions of a madman. And yet who among us can doubt that, when the violence crosses lines of race, precisely such extrapolations would be made?

While we are busy thanking God that a particular psychopath isn't black or white, we might also remind ourselves of the necessity of addressing our national sickness that makes us suppose it would make any particular difference.

At some point, Americans black and white had better come to understand that not every assault by one of them on one of us is an attack by them on us.