NOT EVERY kid who makes mudpies will turn out the sculpture for the Medici Chapel, no. But the road from child's play to deathless art is not so grand a distance as we at first think.

At least this occured to me - I don't say it's true, but it's a thought - during lunch with Eric Rudd, the Washington painter and sculptor, just at the time he was telling me of his vertical toothbrush and non-liquid swimming pool.

"I have no cavities," he observed over the beef at the Jefferson Hotel, "and was explaining to the kids how important it is to brush, and I thought all of a sudden, no wonder people have bad teeth; the toothbrushes are built to go from right to left, instead of being designed to go up and down."

So he devised this T-shaped toothbrush, which even the merest child would automatically use like a window-washing squeegee, brushing up and down. The more he thought of it, the more he thought everybody should have one, so he went down to the Patent Office and looked over all the toothbrushes of yesteryear.

There were some that worked on the same principle as his, but not with the same refinements, and. . .

"So you're making a million on the vertical toothbrushes!" I cried, happy indeed to call Rudd a friend.

"Oh no," he said, getting on with the beef, "I worked it out and saw it would take a lot of money up front, though I did go to Safeway and places like that and saw there were marketing possibilities. But as I saw it, I would have to start out with 200,000 toothbrushes in the bedroom."

And Mrs. Rudd, for some reason, said to hell with that.

One can but wonder how many fortunes have died aborning from the input of "practical" wives.

Rudd also is working on a swimming pool that will be filled with material other than water.

Pellets, I may not divulge their composition, but may say in a nutshell that Rudd is thinking of testing the idea by sawing off the top half of a bedroom door, filling the room with these pellets and jumping in for a swim. I cannot yet report Mrs. Rudd's views on that.

"Can I come swim?" I asked.

"I'm not that far along," he said. "The filtering system stymies me a little. There has to be some way to get the dust out-you know-and stuff that might get into the pool, but it's not impossible to solve."

When the filter sucks out the pellets and blasts them back into the pool, think of the possibilities:

Like a Whirlpool bath, only nice and dry. Think of the uses in medical therapy. Think of-

Rudd has already thought, and he raced along with the idea so that before dessert (which we did not eat) I saw that nobody can live the good life without one of these light-weight non-traumatic waterless pools in the spare bedroom.

And it was here-as Rudd was starting off on some marvelous new material for sculpture, that you just add water and wait 16 seconds and then it swells up very quickly, and you leap to it to start shaping your sculpture-it was just at this point that I saw anew that play and playfulness are the true armature of art.

Plato used to say art comes from God through the artist, much as the Potomac gets into the bathtub through a faucet, and I for one do not deny its divine origins.

Almost everything I know even partially I have learned from years of observing my hounds, and I have long known that they learn almost everything they know (which rude people say is little) from play.

And if that is how dogs learns, who doubts that it's how bipeds do?

Mudpies lead to sculpture; I have always known that. Lipstick on the wall leads to paint. Childish laughter and shrieks lead to the Eastman School of Music, and all art, in brief, springs from the play of children, which in time becomes the play, and the great work of men (and women, of course).

I used to wonder how poetry derived from play. What were the games that resulted in Lear and the Iliad?

And then of course it dawned on me. Literature springs from play that is terminated too soon or frustrated too severely.

Writers are merely people who never had their mudpies, so to speak, and therefor spent the rest of their lives making them in print. I speak here of the grand writers-Homer, Shakespeare, Milne, Cervantes.

Their triumphs, I do not doubt, all sprang from verbalized tears of childhood, reinforced (in the most gorgeous cases) by imagined comforts and "there, there now, it's all right" from God.

There is no other sound explanation for the greatest works, and thus we see that play is the diving board to art. And unless we want a humanity of nothing but writers (which God forbid) we must encourage play.

Several years ago Wilton Dillion, who dreams up seminars and colloquiums for the Smithsonian, was talking about this very thing. As I got it, he was of my opinion, that Sistine ceilings are won on the playing fields of P.S. 106.

So I was not too surprised to learn the Smithsonian is to have a great colloquium on "Play and Inventiveness" next week, one of those invitation-only lectures, with Erik H. Erikson and Joan Erikson as speakers.

Erikson also has the further notion that some young people need "moratoriums," or periods of time lasting a year or two (or more) to get their heads together. Then they go on.

Not only is this notion a spectacular comfort to parents of 20-year olds, it is also perhaps true.

My father used to say you should give a fellow 60 years to get started good, and he was wiser than most.

Lots of play, plenty of rest, time to brood, and the intervention of God: The recipe for art is simple enough. Alas that so often one of the elements of the mix is missing.

And that is why art, when at last it is produced, is prized.