“We are bathing in mystery and confusion on many subjects,” Carl Sagan reflected in an interview in August of 1980, “and I think that will always be our destiny. The universe will always be much richer than our ability to understand it.”

Exactly eight years later, a mind far more scientifically formidable, if not as poetic, ignited in the popular imagination the idea that Sagan’s worldview might be wrong — that the universe might, after all, be fully knowable and fully describable in a single elegant theory.

When Stephen Hawking (January 8, 1942–March 14, 2018) sent his book proposal for what would become A Brief History of Time: From the Big Bang to Black Holes (public library) to Cambridge University Press — his first book of popular science — his editors cautioned him that it contained too much science to be sellable. Every equation, they admonished, would cut book sales in half. Hawking revised the manuscript until it contained a single equation — Einstein’s E = mc². He transmuted all the remaining equations into a scintillating scientific narrative and completed the book just before he rallied Cambridge into a celebration of the 300th anniversary of Newton’s Principia — perhaps the most paradigm-shifting book in the history of science, introducing the theory of gravitation to the world.

A Brief History of Time was published on April Fool’s Day 1988. In the introduction, Carl Sagan extolled Hawking as a “worthy successor” to Newton and lauded the book as replete with “lucid revelations on the frontiers of physics, astronomy, cosmology and courage.” Its accelerated ascent up the bestseller list stunned even Hawking himself. Within months, millions of copies had sold and the book was being translated into multiple languages — success so rapid that it entered the Guinness Book of World Records. (Hawking was amused that his American-made speech synthesizer struggled with the word, pronouncing it Guy-ness.) A “phenomenal international bestseller” for decades to come, in the words inscribed on the cover of the most current edition at the time of Hawking’s death, A Brief History of Time went on to shape the way generations comprehend the universe.

One of the most abiding aspects of the book is Hawking’s succinct description of what makes a good theory. In a sense, it parallels pioneering psychologist Jerome Bruner’s framework of what makes a good story. With an eye to Karl Popper’s famous assertion that “knowledge consists in the search for truth… not the search for certainty,” Hawking writes:

Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.

In her excellent biography, Stephen Hawking: An Unfettered Mind (public library), Kitty Ferguson builds upon Hawking’s formulation of a good theory and offers her own, more expansive and poetic definition:

A theory is not Truth with a capital T, not a rule, not fact, not the final word. You might think of a theory as a toy boat. To find out whether it floats, you set it on the water. You test it. When it flounders, you pull it out of the water and make some changes, or you start again and build a different boat, benefiting from what you’ve learned from the failure.

Some theories are good boats. They float a long time. We may know there are a few leaks, but for all practical purposes they serve us well. Some serve us so well, and are so solidly supported by experiment and testing, that we begin to regard them as truth. Scientists, keeping in mind how complex and surprising our universe is, are extremely wary about calling them that. Although some theories do have a lot of experimental success to back them up and others are hardly more than a glimmer in a theorist’s eyes — brilliantly designed boats that have never been tried on the water — it is risky to assume that any of them is an absolute, fundamental scientific “truth.”

It is important, however, not to dither around forever, continuing to call into question well-established theories without having a good reason for doing so. For science to move ahead, it is necessary to decide whether some theories are dependable enough, and match observation sufficiently well, to allow us to use them as building blocks and proceed from there. Of course, some new thought or discovery might come along and threaten to sink the boat.

Hawking’s own life was animated by one particular theoretical pursuit — the search for a theory, colloquially known as a “theory of everything,” that unifies Einstein’s general relativity, the gravity-based science of the very large, and quantum mechanics, the science of the very small, based on three non-gravitational forces: the weak, strong, and electromagnetic forces. In the penultimate chapter of A Brief History of Time, Hawking considers this unholy grail of physics against the backdrop of the history of science:

The prospects for finding such a theory seem to be much better now because we know so much more about the universe. But we must beware of overconfidence — we have had false dawns before! At the beginning of this century, for example, it was thought that everything could be explained in terms of the properties of continuous matter, such as elasticity and heat conduction. The discovery of atomic structure and the uncertainty principle put an emphatic end to that. Then again, in 1928, physicist and Nobel Prize winner Max Born told a group of visitors to Göttingen University, “Physics, as we know it, will be over in six months.” His confidence was based on the recent discovery by Dirac of the equation that governed the electron. It was thought that a similar equation would govern the proton, which was the only other particle known at the time, and that would be the end of theoretical physics. However, the discovery of the neutron and of nuclear forces knocked that one on the head too. Having said this, I still believe there are grounds for cautious optimism that we may now be near the end of the search for the ultimate laws of nature.

Complement the timelessly fascinating Brief History of Time with poet Marie Howe’s stunning tribute to Hawking, Hawking himself on the meaning of the universe, his “theory of everything” animated in 150 seconds, the story of how his mother shaped his genius, and the children’s book about time travel he co-wrote with his daughter, then revisit some of today’s leading thinkers on the most elegant theory of how the world works.