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and did not require explanation.

His understanding of explanation did not depart far from the modern philosophical mainstream, though its jargon of explanans and explanandum was an alien language to him. Like most philosophers, he found explanations most satisfactory when they cal ed upon a generalizing, underlying “law.” A thing is the way it is because other things of its kind are al that way. Why does Mars travel around the sun in an el ipse? Feynman explained—and ventured deep into philosophical territory—in an invited lecture series at Cornel University in 1964. He began by speaking, nominal y, about the law of gravitation. In reality his subject was explanation itself.

Al satel ites travel in el iptical orbits. Why? Because objects tend to travel in a straight line when left alone (the law of inertia) and the combination of that unchanging motion and a force exerted toward a center of gravity—by the law of gravitation—creates an el ipse. What validates the law of gravitation? Feynman expressed the scientist’s modern view, a blend of the pragmatic and the aesthetic.

He cautioned that even so beautiful a law was provisional: Newton’s law of gravitation gave way to Einstein’s, and a necessary quantum modification eluded physicists even now.

That is the same with al our other laws—they are not exact. There is always an edge of mystery, always a place where we have some fiddling around to do yet.

This may or may not be a property of Nature, but it

certainly is common to al the laws as we know them today.

Yet in its unfinished form the law of gravitation explained so much. To a practicing scientist, that validated it. The same smal parcel of mathematics explained Tycho Brahe’s nightly observations of the planets in the sixteenth century and Galileo’s measurements of bal s rol ing down inclined planes, timed against the beat of his own pulse. The planets are fal ing, Newton reasoned; the moon feels the same force as an earthly projectile, the force weakening with the square of the distance. A law is not a cause —

philosophers stil wrestled with this distinction—yet it is more than merely a description. It precedes the thing explained, not in time but in generality or in profundity. The same law explained the earth’s symmetrical y bulging tides, rising both toward and away from the moon, and the newly measured orbits of the moons of Jupiter. It made new predictions that scientists could confirm or disprove with experiments on bal s hanging delicately in a laboratory or observations of majestical y rotating galaxies a hundred mil ion mil ion times larger. “Exactly the same law,”

Feynman said, and added—having struggled to find the right wording—

Nature uses only the longest threads to weave her pattern, so each smal piece of the fabric reveals the organization of the entire tapestry.

Meanwhile, why does an object in motion tend to travel forever in a straight line? That, Feynman said, nobody knows. At some deep stage, the explanations must end.

“Science repudiates philosophy,” Alfred North Whitehead had said. “In other words, it has never cared to justify its truth or explain its meaning.” Feynman’s col eagues liked to think of their gruffly plain-spoken pragmatist hero as the perfect antiphilosopher, doing rather than justifying. His own rhetoric encouraged them. He lacked patience for the now-popular What is reality? brand of speculation arising from quantum-mechanical paradoxes. Yet he could not repudiate philosophy; he had to find ways to justify the truth that he and his col eagues sought. The modern physics had banished any possibility of discovering a system of laws unambiguously tying effects to causes; or a system of laws deduced and conjoined with perfect logical consistency; or a system of laws rooted in the objects that people can see and feel. For philosophers, these had al been marks of a sound explanatory law. Now, however, a particle might or might not decay, an electron might or might not pass through a slit in a screen. A minimum principle like the principle of least action might be derived from laws of forces and motion, or those laws might depend on the principle: who could say with logical certainty? And the basic stuff of science had grown inexorably more abstract.

As the physicist David Park put it: “None of the entities that appear in fundamental physical theory today are accessible to the senses. Even more … there are phenomena that

apparently are not in any way amenable to explanation in terms of things, even invisible things, that move in the space and time defined by the laboratory.” With al these traditional virtues removed—or worse, partly removed while stil partly necessary—it fel to science to build a new understanding of the nature of explanation. Or so Feynman argued: the philosophers themselves, he said, were always a tempo behind, like tourists moving in after the explorers have left.

Scientists had their own forms of blindness. It was often said in the quantum-mechanical era—Feynman had said it himself—that the only true test of a theory was its ability to produce

good

numbers,

numbers

agreeing

with

experiment. The American pragmatism of the early twentieth century had brought forth views like Slater’s at MIT: “Questions about a theory which do not affect its ability to predict experimental results correctly seem to me quibbles about words.” Yet Feynman now felt a hol owness in the purely operational view of what a theory means to a scientist. He recognized that theories came laden with mental baggage, with what he cal ed a philosophy, in fact.

He had trouble defining this: “an understanding of the law”;

“a way that a person holds the laws in his mind …” The philosophy could not be discarded as readily as a pragmatic scientist might suggest.

Consider a Mayan astronomer, he suggested. (In Mexico he had grown interested in the deciphering of the great ancient codices, hieroglyphic manuscripts that employed long tables of bars and dots to set down an intricate

knowledge of the movements of sun, moon, and planets.

Codes, mathematics, and astronomy—eventual y he delivered a lecture at Caltech on deciphering Mayan hieroglyphics. Afterward, Murray Gel -Mann “countered,”

Feynman said, with a series of six lectures on the languages of the world.) The Maya had a theory of astronomy that enabled them to explain their observations and to make predictions long into the future. It was a theory in the utilitarian modern spirit: a set of rules, quite mechanical, which when fol owed produced accurate results. Yet it seemed to lack a kind of understanding. “They counted a certain number and subtracted some numbers, and so on,” he said. “There was no discussion of what the moon was. There was no discussion even of the idea that it went around.”

Now a “young man” approaches the astronomer with a new idea. What if there are bal s of rock out there, far away, moving under the influence of forces just like the forces that pul rocks to the ground? Perhaps it would make possible a different way of calculating the motions of the heavenly bodies. (Feynman certainly had memories of a young man confronting his elders with new, half-formed physical intuitions.)

“Yes,” says the astronomer, “and how accurately can you predict eclipses?” He says, “I haven’t developed the thing very far yet.” Then says the astronomer, “Wel , we can calculate eclipses more accurately than you can with your model, so you must

not pay any attention to your idea because obviously the mathematical scheme is better.”

The notion that alternative theories could account plausibly for the same observations had slipped into a central position in the working philosophy of scientists.