For Pavlov, the pieces — the atoms — were the simplest measurable associations. Stimulus-response, stimulus-response, stimulus-response. By weaving together webs of these connections, he hoped to capture the ineffable.

“The movement of plants toward the light and the seeking of truth through a mathematical analysis — are these not phenomena belonging to the same order?” he wondered. “Are they not the last links in an almost endless chain of adaptabilities which appear everywhere in living creatures?” We expect this kind of grand philosophical pronouncement more from a William James than from an Ivan Pavlov. That is how poorly the man has been understood.

After reading about the case of Anna O., made famous by Sigmund Freud, Pavlov began contemplating neurosis in a dog. Freud believed that Anna’s condition — hysteria, it was called back then — arose from the stress of caring for her dying father. She was devastated by his plummeting health yet determined to repress her grief and maintain a cheerful face. The result of these opposing psychological forces, as Freud saw it, was a nervous breakdown.

Pavlov thought he recognized a similar phenomenon in a dog named Vampire. The animal had been trained, through salivation experiments, to react differently to two images: an ellipse and a circle. One shape would be reinforced, the other suppressed. As the ellipses were made increasingly rounder and less oval-like, the task grew harder until finally Vampire could not tell the two shapes apart.

And so the poor dog snapped. Originally calm by nature, he began yelping and running in circles, habitually barking for no apparent reason and drooling copiously. Like Anna O., he was caught between two impulses — excitation from the circle and inhibition from the ellipse.

As his theories developed, Pavlov proposed that behavior in dogs and in people could be explained through half a dozen such processes. But he was soon overwhelmed by the complications. Even dogs, he came to realize, had different personalities. Early on, he counted three “nervous types,” a number that later grew to more than 25.