One reason Ronald Reagan is president today is that Jimmy Carter seemed a bit penitential about American prosperity, a shade too receptive to "Third World" complaints (of the sort soon to be aired at Cancun in the "north-south dialogue" recommended by the Brandt Commission) about economic colonialism.

As his Philadelphia speech shows, Reagan is not. His message for the petitioners at Cancun is that the southern hemisphere should clear its collective mind of the colossus-of-the-north mythology and, instead, view the United States as a model example of bootstrap economics. In his view, our history shows how a people beginning with nothing can prosper by hard work and a climate of freedom. "We Americans" he said, "can speak from experience on this subject. Few countries were less developed than we were when the original settlers arrived here."

That is all very true, of course, if one is thinking of the privations of Plymouth and Jamestown, although the links that stretched back across the Atlantic to the chartered companies and countinghouses of London seemed to have been overlooked.

All of us tend to use history selectively. But surely our presidents could find a middle way between breast-beating and smugness when they address the attitudes of the "developing" nations, if only because few of those nations have a virgin continent to plunder. If Jimmy Carter leaned to breast-beating, Reagan leans, I think, to smugness.

And it is a historically selective smugness. As southern Americans know, there is a suggestive resemblance between the situation their great- grandfathers faced (or thought they faced) a century or so ago and the situation the "Third World" countries face (or think they face) today.

Whatever layer of that defeated society they came from or spoke for, almost all of them imagined themselves the victims of a colonial system. And it tended to make them angry and obstreperous. They knew, or thought they knew, that 19th century American industrial prosperity owed a great deal to British capital, to lavish distribution of cheap public land in the West and to a government in Washington that not only took a tolerant view of capitalism but zealously crushed reformist efforts within the states to control its harsher consequences for laborers, immigrants and Indians.

As they saw it, or thought they saw it, the South as an exporter of raw commodities stood at a perennial disadvantage because of protective tariffs and freight-rate differentials.

Revisionist historians have become critical in recent years of the underdog creed that was so much a staple of the South's "Third-World" ideology between the Civil War and the New Deal. We smile fondly today at the recollection of Henry Grady's famous oration, the one in which he reflected, of a Georgia funeral, that Georgia had provided only the corpse and the hole in the ground--all else being "imported" on disadvantageous and exploitative terms. In any case, no one in the booming Sun Belt cities worries about such old memories these days.

But there is perhaps a lesson in these memories for presidents. It may be wise to recall, as Vann Woodward reminded us in his essay, "The Search for Southern Identity," that the bounty Americans think of as their hard-earned heritage is atypical of the world's common experience, and once had its critics here at home among those who felt left out.

In the last generation, an element of becoming modesty about American prosperity was afforded by southerners of the more reflective and history-minded sort. They remembered how their ancestors had felt. And those ancestors had felt about the government in Washington very much as people in the slums of central Africa must feel about it today.

Reagan is certainly right to think that our critics in Cancun need reminding that some of their attitudes are ideological and misplaced, and to a degree disabling.

But Reagan and his spokesmen, by remembering their American history a bit less selectively, might also temper the sharp advice with some recognition that others, too, including at one time a number of southern Americans, felt as the plaintiffs of Cancun feel today.