During the Enlightenment, every fixed point of knowledge began to wobble. Illustration by Brian Cronin

We like to think of ourselves as living in an age of unprecedented disruption. Just look at all the commonplace features of our world that didn’t exist a century ago—jet travel, television, space flight, the Internet. If you could transport someone from the year 1916 to the present, we ask a little proudly, wouldn’t that person be stupefied by the changes? And, of course, he would be, at least for a few days, until he figured out how everything worked. But one thing would be very familiar to such a time traveller: the pride, and the anxiety, we feel about being so modern. For people in the early twentieth century were as acutely aware of their modernity as we are of ours, and with just as good reason. After all, they might have said, imagine someone transported from 1816 to 1916: what would that person have thought of railroads, telegraphs, machine guns, and steamships?

Modernity cannot be identified with any particular technological or social breakthrough. Rather, it is a subjective condition, a feeling or an intuition that we are in some profound sense different from the people who lived before us. Modern life, which we tend to think of as an accelerating series of gains in knowledge, wealth, and power over nature, is predicated on a loss: the loss of contact with the past. Depending on your point of view, this can be seen as either a disinheritance or an emancipation; much of modern politics is determined by which side you take on this question. But it is always disorienting.

If we are looking for the real origins of the modern world, then, we have to look for the moment when that world was literally disoriented—stripped of its sense of direction. Heliocentrism, the doctrine that the earth revolves around the sun rather than vice versa, was announced by Copernicus in 1543 and championed by Galileo in the early sixteen-hundreds. This revelation was immediately experienced as a profound dislocation, as John Donne testified in his 1611 poem “An Anatomy of the World”: “The sun is lost, and th’ earth, and no man’s wit / Can well direct him where to look for it.” More than two hundred and fifty years later, Nietzsche was reeling from the same cosmic loss of direction: “What were we doing when we unchained this earth from its sun? . . . Are we not plunging continually? Backward, sideward, forward, in all directions? Is there still any up or down?” Modernity is a vertigo that began in the sixteenth century and shows no sign of letting up.

Nietzsche is usually classified as a philosopher, Donne as a poet, and Galileo as a scientist. But one of the premises of Anthony Gottlieb’s new book, “The Dream of Enlightenment” (Liveright)—the second installment of his lucid, accessible history of Western philosophy—is that thought cannot be divided according to disciplines in this way. For philosophy, in particular, such a division is misleading. Today, we tend to think of philosophy as a specialized academic pursuit: a philosopher is a professor of philosophy. But none of the founders of modern philosophy whom Gottlieb discusses fit that description. Some were mathematicians: René Descartes invented the Cartesian coördinate system with its x- and y-axes, and Gottfried Leibniz invented calculus (around the same time as, but independently of, Isaac Newton). Some were professionals: Baruch Spinoza ground lenses for optical equipment; John Locke was a doctor and a diplomat. And some were literary writers, like David Hume, who was better known in his lifetime for his “History of England” than for his philosophical works. Usually, they overlapped several categories.

One of Gottlieb’s central insights is that, as he wrote in his previous volume, “The Dream of Reason,” which covered thought from the Greeks to the Renaissance, “the history of philosophy is more the history of a sharply inquisitive cast of mind than the history of a sharply defined discipline.” You might say that philosophy is what we call thought in its first, molten state, before it has had a chance to solidify into a scientific discipline, like psychology or cosmology. When scientists ask how people think or how the universe was created, they are addressing the same questions posed by philosophy hundreds or even thousands of years earlier. This is why, Gottlieb observes, people complain that philosophy never seems to be making progress: “Any corner of it that comes generally to be regarded as useful soon ceases to be called philosophy.”

Therefore, philosophy shouldn’t be considered a kind of centuries-long chess match, with thinkers taking turns in an abstract intellectual game. For instance, in treating the philosophy of the seventeenth and eighteenth centuries, it is conventional to cast it as a struggle between “rationalists” and “empiricists.” In this account, everyone from Descartes to Hume is engaged in one long battle over whether truth is to be found “in here,” through strictly logical reasoning on the model of mathematics, or “out there,” through observation of the world. This debate, in turn, was finally resolved by Immanuel Kant, in the late eighteenth century, when he figured out a way to show that both sides were correct, since all perception is necessarily filtered through the categories imposed by our minds.

There is some truth to this account—the origin of knowledge was certainly a concern for all these thinkers. But Gottlieb, who is not an academic and spent much of his career as a journalist—he is a former executive editor of The Economist—sees that they were situated in a much wider world. Their thought was informed not just by previous philosophy but by politics, religion, and science—the whole intellectual and spiritual life of their times. And it was because these times were so tumultuous that they were able to think in such a radical way. Eras in which everything is up for grabs are very rare, and they seem to be highly productive for philosophy. As Gottlieb points out, much of the Western philosophy that still matters to us is the product of just two such eras: Athens in the fifth and fourth centuries B.C. and Western Europe in the seventeenth and eighteenth centuries A.D.

It is hard for us to comprehend how totally Western consciousness was transformed during the second of these two periods, precisely because we live in its aftermath. In just a few generations preceding it, every fixed point that had oriented the world for thousands of years began to wobble. The discovery of America destroyed established geography, the Reformation destroyed the established Church, and astronomy destroyed the established cosmos. Everything that educated people believed about reality turned out to be an error or, worse, a lie. It’s impossible to imagine what, if anything, could produce a comparable effect on us today. Even the discovery of alien life in the universe wouldn’t do it, since we have long learned to expect such a discovery, whereas medieval Europeans could never have anticipated the existence of America, or of electricity.

Perhaps if it were somehow confirmed that, as some thinkers speculate, our universe is actually a simulation run on a computer by an unfathomably advanced intelligent civilization, we would feel an analogous sense of confusion and possibility. That would raise the questions that were at the heart of philosophy in both of Gottlieb’s magical periods in a new way. What does it mean for something to be? Why does anything exist in the first place? Such metaphysical questions are what, from the very beginning, gave philosophy a bad name, because to practical-minded people they appear useless. That is why the comic playwright Aristophanes, in his play “The Clouds,” portrayed Socrates as discussing questions such as whether a gnat buzzes through its nose or its anus. No one knows, sure, but also no one cares.