“How does Harold draw himself?” my friend Noam wondered. Noam is 4, so he has a taste for the kind of bracing philosophical speculation that “Harold and the Purple Crayon” — the classic children’s book about a young boy and his magical marker — invites, along with wonder, mischief and delight.

There are grown-ups who might answer that Harold obviously doesn’t draw himself, since he’s the only figure in the book not rendered in a thick purple line. But Noam has a point. Harold is the sole visible creator in his universe. The pages he inhabits are plain white expanses until he fills them: with simple objects like a half-moon, a window frame and a sailboat; with more elaborate compositions like a high-rise cityscape, an apple tree guarded by a dragon and a seaside picnic featuring a moose, a porcupine and nine kinds of pie.

Given such effortless, abundant inventiveness, it’s reasonable to surmise that this industrious and unflappable child might indeed be his own creation. At least in “Purple Crayon,” the first of seven books about Harold, there is no sign of a parent, though Harold does have a home and a bed. (A mother is mentioned in his second adventure, “Harold’s Fairy Tale.”) Even things that might have already been there seem to flow from his crayon, which underlines the paradox that captivated Noam. Harold makes his world. Who made Harold?

While I’ve always been charmed, and sometimes unnerved, by the deep metaphysics of children’s literature, I’m more comfortable dealing with empirical, historical matters. So for the moment I’ll dodge my friend’s query. Harold is the brainchild of Crockett Johnson, who was born in 1906 and took up picture books in midlife, after a career as a design director and political cartoonist. But Johnson himself was to some degree self-invented. The name on his birth certificate is David Leisk; the name on the books was not so much a pseudonym (Johnson was his middle name, Crockett a childhood nickname) as a persona, just as “Seuss” (also a middle name) was for his contemporary Theodor Geisel.

Their careers followed similar paths. Like Geisel, Leisk spent much of the 1930s and early ‘40s working in advertising and lending his graphic talents to antifascist and left-wing causes. Unlike Geisel, he drew a syndicated daily comic strip, “Barnaby,” which ran from 1942 to 1947. The titular character is a sensible, self-sufficient 5-year-old with an active imagination and an exasperating imaginary friend (or “fairy godfather”) called Mr. O’Malley. A forerunner of both Charles Schulz’s “Peanuts” (for its unsentimental child’s-eye view of the world) and Walt Kelly’s “Pogo” (for its satirical pokes at contemporary reality), “Barnaby” counted Dorothy Parker, Duke Ellington and W.C. Fields among its fans.

Harold first appeared in 1955 and has been a familiar presence on bookshelves ever since. He is not quite as famous as the Cat in the Hat, who arrived two years later; he is certainly less of a show-off. Johnson, far less prolific than Seuss, never became a household name or a pop-culture brand. Harold is another story. He appeared in a series of whimsical animated shorts on HBO in the early 2000s, and his influence has been felt across children’s publishing and beyond, touching “The Simpsons,” “Captain Underpants” and the music of Prince. The character is making his feature film debut this summer as a full-grown man-child played by Zachary Levi in a hybrid live-action/animated movie directed by Carlos Saldanha, a veteran of the “Ice Age” and “Rio” franchises.

Johnson died in 1975, Seuss in 1991. Neither lived to see the 21st-century boom in children’s visual entertainment. Both were crucial figures in what might be described as the midcentury modernist generation of children’s book creators, a loose affiliation of writers, illustrators, editors and librarians who shook loose the didactic, Victorian assumptions of the genre and transformed it into the vibrant and protean art form we now take for granted. Margaret Wise Brown, author of “Runaway Bunny” (1942) and “Goodnight, Moon” (1947), was one of the movement’s pioneers. So was Ruth Krauss, who married Johnson in 1943 and whose second book, “The Carrot Seed,” was published with his illustrations in 1945. The pair — subjects of a lively and affectionate double biography by Philip Nel — were something of a kid-lit power couple, serving as mentors, collaborators and sounding boards for others in the field.

For “A Hole Is to Dig,” her whimsical 1952 “book of first definitions,” Krauss enlisted the talents of a young Maurice Sendak, who cited her as an important influence and role model. According to him, she was “the first to turn children’s language, concepts and tough little pragmatic thinking into art.”

That describes Johnson’s approach, too, though his emphasis is graphic rather than linguistic. Harold does not speak; the books are entirely free of dialogue, and his escapades are recounted in a matter-of-fact narration leavened by bubbles of wit. In “Harold’s Trip to the Sky,” he finds himself in a desert without much to do “except maybe play in the sand.” But “then he remembered how the government has fun in the desert. It shoots off rockets.”

He draws one of those, but leaves “the government” to the reader’s imagination. From time to time, Harold will sketch a figure of adult authority — a policeman in “Purple Crayon,” a morose king in “Harold’s Fairy Tale” — but they are benignly comical features in the landscape, neither imposing nor especially useful. The policeman, conjured to help Harold find his way home, “pointed the way Harold was going anyway. But Harold thanked him.”A child at play is also at work: solving problems, making discoveries, responding to challenges of his own devising. Harold clearly enjoys what he does — he is often smiling, his eyes wide — but he also takes his job seriously. His face is remarkably expressive, but the range of his expression is narrow: quiet pleasure, mild puzzlement, skeptical amusement. He is a creature of nuance. He is an artist.

Johnson almost always shows him in profile, conveying his absorption in the endless task before him, which is also Johnson’s task: drawing. Harold makes up the world as he goes along, and does his best to represent it faithfully — to obey its laws of time and space, cause and effect, even as he rewrites them according to his own needs and whims. Harold is capable of surprising himself, making mistakes and correcting them on the fly. He tumbles off the end of a line and falls through space, rescuing himself with a hot-air balloon. His rocket misses the moon “by a mile,” so he sketches Mars. A church steeple turns out to be the pointed hat of “a GIANT WITCH,” whom he chases away with a swarm of mosquitoes. He repurposes an ominous flying saucer as a bowl of oatmeal.

Through it all, Harold moves forward, from left to right, from one page to the next, following a thread of constant experimentation, revising his own story until it leads him home to a hot meal, a bedtime story or a good night’s sleep.