Philosophy is many things, Gottlieb suggests, including much that we no longer call philosophy, but one of its recurring features is what William James called “a peculiarly stubborn effort to think clearly.” As Gottlieb declares in the first volume, the idea of clarity has not always seemed foremost, but the stubbornness is everywhere. “The attempt to push rational inquiry obstinately to its limits” is the name of the project. Sometimes it fails entirely, and the dream “seems merely a mirage.” At other times, though, “it succeeds magnificently, and the dream is revealed as a fruitful inspiration.” The dream appears as either fantasy or revelation, and ­Gottlieb skillfully tells “both sides of the story.” But what about the monsters?

Gottlieb reminds us that, for Bertrand Russell, Rousseau was responsible for the rise of Hitler, because his idea of a general will “made possible the mystic identification of a leader with his people.” Leibniz was inclined “to confuse his own mind with that of God.” Descartes “was too quick to assume that whatever seemed to him to be necessarily true was in fact so.” Hobbes was “almost charmingly naïve” about the supposed rationality of sovereigns with absolute power. This last instance becomes especially strange when we think of Hobbes’s eloquent elaborations of what people are like when left to their own devices (“no Knowledge of the face of the Earth; no account of Time . . . no Society; and which is worst of all, continuall feare, and danger of violent death”) but then, as Gottlieb shrewdly says, Hobbes “wanted above all to scare people by stressing the anarchy that would prevail in the absence of government.” He could idealize government on the same pretext.

Gottlieb is fully aware of the monsters in the dream, but doesn’t allow them to dominate his book. He is committed to the positive aspects of inquiry, especially where scientific advances are involved. “It is by virtue of its engagement with the special problems posed by modern science that modern philosophy is distinguished from premodern philosophy.” Gottlieb often makes fun of his philosophers, but gently, as a way of bringing us closer to them, and they emerge as brilliant, vulnerable humans rather than monsters of any kind. Descartes worried about “the divine insurance plan”; “Hobbes got rather carried away” when he told us how solitary, poor, nasty, brutish and short life was. “If Leibniz had been a composer, most of his symphonies would have been unfinished.”

Descartes gets a slightly harder ride than the others, and Gottlieb seems to have changed his mind about him since he wrote the earlier book. There his writings were described as “engaging,” and now they appear as “dubious” and “built on sand,” with Descartes himself accused of “trying to work out too much in his head.”

This last remark looks like a rather odd verdict on a philosopher, but it makes sense in the context of the book, and of course Gottlieb is not denying Descartes’s immense influence. All of Gottlieb’s chief subjects — Descartes himself, Hobbes, Spinoza, Locke, Leibniz, Hume — are engaged, precisely and in a new way, with the world outside the head. Even geometry led to politics and social theory; advanced theoretical thought constantly engaged with the physical and mechanical sciences — for a long time these disciplines were still housed under the name of philosophy. There was plenty of room for work inside the head, of course, and as Gottlieb says, “philosophers always travel in several directions at once,” but the material world was a laboratory and an authority replacing, even for religious thinkers, the old, unappealable orders of the church.