A mere 0.6% of genetic material separates us from our closest cousins in the animal kingdom. The figure seems tiny, but the difference is immense. For centuries, philosophers distinguished humans from animals on the basis of our rational faculty. But we know now that certain animals — not only apes but rats as well — possess a kind of inductive reasoning that enables them to manipulate levers or flip switches to reach a remote banana or a hidden nut. Language doesn’t explain our differences.

Animals may lack grammatical notions but some overachieving chimps have mastered the vocabulary of a human toddler and parrots can put sailors to shame with their salty invectives. Theologians have held that the distinction lay in the soul: Humans had immortal souls, animals did not. But if the “eyes are windows of the soul,” as these same theologians taught, then one glimpse of a llama’s soulful gaze or an eyeball encounter with a noble pug should be enough to blur, if not refute, such flimsy distinctions.

In “The Human Animal in Western Art and Science” (University of Chicago Press, 320 pages, $40), the Oxford art historian Martin Kemp offers a new solution. The book is based on the Louise Smith Bross Lectures that Mr. Kemp gave at the Art Institute of Chicago in April 2000 and that he has revised and expanded, supplementing his witty and erudite text with some 185 marvellous illustrations. His theme is “humanized animals and animalized humans” and he ranges widely to explore it. Beginning with a lucid (and rather gruesomely illustrated) discussion of the four humours, which humans and animals were thought toshare, Mr. Kemp moves through the centuries. Dürer, Cranach, Da Vinci, and Rembrandt may occupy pride of place, and rightly so, but many fascinating, lesser known figures appear as well. These include the brilliant Charles Le Brun in 17th-century France, whose drawings of human facial expressions from despair to astonishment are one of the marvels of the volume, as well as the halfmad Viennese sculptor Franz Xaver Messerschmidt, whose contorted portraits of “manic grins” and the grimaces of “beak-like mouths” fairly leap from the page. In such depictions, humans are animalized and animals humanized, so disturbingly that all our artificial boundaries begin to dissolve.

Although Mr. Kemp is steeped in the works of the great masters of Western art, he has an endearing taste for kitsch that he draws on to enliven his discussion. He devotes a fine chapter to early automata, such as Jacques Vaucanson’s mechanical “digesting duck” which not only ate grain and splashed in water but “passed his excrements, flapping and spreading his wings.” He enlists P.T. Barnum and his lurid sideshows and casts sidelong, but affectionate, glances at Alfred Hitchcock’s “The Birds” and Bela Lugosi’s “Dracula,” even finding a place for Raquel Welch, as a busty Neanderthal in a rawhide bikini, in the 1966 flick “One Million Years B.C.” Through such images, high and low, Mr. Kemp illumines the shadowy interchanges between the realms of man and beast to show, yet again, that however parallel they may seem, they constantly intersect.

Animals have of course figured strongly in literature. Mr. Kemp’s treatment of various speaking beasts, from works by Aesop to Kipling, are as astute as his analyses of visual art. His comments on the “Fables” of La Fontaine, an ardent opponent of Descartes’s notion of animals as “mere machines,” are especially perceptive. He has a gift for bringing important but half-forgotten artists and thinkers back to life, such as Jean-Baptiste Oudry, greatest of animal portraitists, or Georges-Louis Leclerc, better known as the Comte de Buffon, not only the most learned naturalist of the 18th century, but a superb prose stylist. There are a few oversights. Mr. Kemp might have included Franz Kafka, the most inventive animal fabulist of the last century, whose “Report for an Academy” vividly shows what miseries befall an innocent ape when he “evolves” into a man.

Mr. Kemp’s new solution proposes to draw a line between man and beast not on the basis of reason or the presence of a soul, but in accord with a subtler distinction. Although some animals use crude tools, no animal uses what he calls “indirect tools.” These are tools, such as a needle or a bow and arrow, which require a series of imaginative “pre-visualizations,” both to invent and to use.

To conceive a needle, one must be able to envisage the process of connecting two pieces of material; this in turn involves picturing such implements as thread or the needle’s eye and, in a further stage, the specific looping motion of sewing. As in chess, the ability to picture objects and processes in future and successive stages is required. This seems clearly beyond the capacity of any animal.

Mr. Kemp’s argument is persuasive. Such strategic visualization, proceeding by a logic of images rather than of concepts, does appear peculiarly human. And yet, the puzzle of our apartness remains; to be human is to be caught in a strange midway kingdom. We sense but can’t know the minds of our fellows in that neighboring realm. Montaigne put it best, in a remark Mr. Kemp quotes: “When I play with my cat, how do I know that she is not passing time with me rather than I with her?”

eormsby@nysun.com