On the morning of what was to be a momentous day, I decided to dip a toe into the rushing stream of the modern world by seeing the movie "Flashdance." Little did I then suspect that this movie, about which the intelligentsia has been quite rude, actually unlocks the mystery of the human race's place in the cosmos.

It is about a young lady welder (no kidding) in Pittsburgh who in the evenings performs in a blue-collar bar where she dances like a dervish and twists her comely self as though she is auditioning for the role of a soft pretzel.

Like "Breaking Away," which was set in Bloomington, Ind., and "Personals," which was set in Minneapolis-St. Paul, and "Diner," which was set in Baltimore, "Flashdance" is almost lyrical about a place that does not often evoke lyricism. ("O, Pittsburgh"? "Let us now praise Bloomington?")

Our lady welder is to her gender, and her end of Pennsylvania, what "Rocky" was to Philadelphia. She is a monument to upward mobility through sweat. By grit and pluck (and pumping iron) she dances her way into, yes, of course, our hearts, but also into ballet school. In the last scene she is in the arms of her lover, another upwardly mobile type who has risen from a rough neighborhood to a Porsche, and owns the dark, satanic mill where our lady welds.

Her language is, I gather, a badge of emancipation these days. That is, it would cause blushes beneath deck in a troop ship. But her clothes have become a commercial force. In the Juniors department at Woodward & Lothrop in Chevy Chase and across this broad land, you can buy the "Flashdance" look.

When our heroine wends her weary way home after a hard day over the acetylene torch, she slips into something . . . well, "comfortable" hardly does justice to it. It is so loose she almost slips right out again. The Flashdance look is a ragged sweatshirt hanging off one fetching shoulder and barely hanging on the other. A tear or two is required, and at stores that know their stuff you can now buy pre- torn and elegantly unfinished garments. For halfhearted flashdancers, some garments come equipped with snaps that can snap up the torn look and make the thing whole for, I guess, formal occasions.

What is the world coming to? To a place it has been before. In his new book "Lost in the Cosmos: The Last Self-Help Book," Walker Percy recalls that when Wallis Warfield Simpson appeared at Ascot with the second button of her blouse inadvertently unbuttoned, lots of women began leaving their second buttons unbuttoned. And when John Wayne was filming "Red River," and his belt buckle slipped to one side, lots of men slipped their buckles sideward.

Cosmos? I thought we were in places like Pittsburgh. Percy says: Pittsburgh, cosmos, what's the difference? The fading of religious explanations of mankind's place in the cosmos has left the self dislocated and without identity. So people put on new identities-- Mrs. Simpson's, or John Wayne's, or our lady welder's.

Liberated by skepticism from the restraints of religion, by democracy from social oppression, by technology from drudgery, the modern individual is free to do as he or she pleases. And what does it please him or her to do? Unbutton the second button, slip the belt sideward, don a pre-torn jersey.

This is not to say that modern life is problem-free. Percy cites this letter to Dear Abby: "I am a 23-year-old liberated woman who has been on The Pill for two years. It's getting pretty expensive and I think my boyfriend should share half the cost, but I don't know him well enough to discuss money with him."

That lady may be lost in the cosmos, and so may we all be, whether we know it or not. But the lady welder, unlike those derivative selves who want to dress like her, is comfortable in the cosmos, and not just because her clothes are so comfortable.

She is like two persons Percy mentions--Franz Schubert, who sat in beer halls writing lieder on the tablecloths, and Pablo Picasso, who sat in restaurants molding animals out of bread. She is so totally absorbed in a vocation --both a gift and a mastering passion --that she has no time to be absorbed with the self's worries about itself. And that is the moral of the story: you can pursue happiness by wearing a torn jersey. You can catch it by being good at something you love.

You thought "Flashdance" was just music? It is metaphysics, of which they have some in Pittsburgh.