It is a brilliant idea to compare and contrast these two philosophers - not only in respect of their ideas, but also in respect of their personalities, life-styles and the historical settings in which they operated. They are both very difficult philosophers, and it is one of the many virtues of this sparkling book that they are made as accessible to the general public as they can be. Even so, the relevant passages will still be rather hard going for readers new to the ideas. Particularly close reading is required for chapter 16 near the end of the book, in which Stewart shows that Leibniz was entangled with Spinozism even when the differences between the two men's philosophies appear at their starkest.
As for the description of their personalities, they come to life in the most vivid way. The different sides of Spinoza are arrestingly described, as is the vanity, the restless and pushy worldliness and the basic insecurity of Leibniz, of whose varied secular career we are also given an entertaining account.
Leibniz was a polymathic and imaginative thinker, but Stewart's picture of him leaves one with the impression that, especially in his relationship with Spinoza, he was thoroughly duplicitous: flattering in his correspondence with him, but denouncing him in letters written to others. Stewart plays fair and provides what excuses he (and other authors) can find for Leibniz (pp. 114 to 119), but there is no doubt that Spinoza emerges from his pages as much the more admirable, honest, austere and courageous human being.
In 1670 Spinoza had published his Tractatus Theologico-Politicus, which caused such a European-wide storm of obloquy that he had arranged for his other books and papers to be published only after his death. Among these papers were letters he had received from Leibniz, and Leibniz was now terrified that their publication would compromise him: not simply because he had been in correspondence with Spinoza after the publication of the Tractatus and had even visited him for several days in 1676, just four months before Spinoza's sudden death, but also because Leibniz's papers show a constant battle within himself: there was so much of Spinoza's thought which he found persuasive, and yet so much which he found undermining not only the orthodox idea of God, but, he thought, the very basis of morality. In his later writings Leibniz occasionally confessed that he had once been tempted by Spinoza's ideas, but it became an obsession with him to brand Spinoza as a dangerous atheist and to ascribe non-existent Spinozist views to such as Isaac Newton and John Locke.
Leibniz thought that belief in a personal and benevolent God and in the immortality of the soul was necessary for human well-being and happiness; but, as Stewart several times points out, it was the beliefs themselves rather than their truth that mattered to him. He does not in fact seem to have been a very religious person himself: his faithful assistant Eckhart said that in the 19 years during which they worked together, he rarely saw him in church and never saw him take communion. And on his deathbed he refused the Last Sacraments.
I cannot help coming away from this book with the idea that not only was Spinoza by far the greater personality of the two, but also the clearer thinker. In grappling with Spinoza, Leibniz had to engage in intellectual fancies and contortions that seem to me totally absurd. I am probably missing something, since Bertrand Russell would call Leibniz "one of the supreme intellects of all time", and Stewart's own concluding pages express a sympathy for what Leibniz was all about which I cannot share.
The irony is that Leibniz was so frightened by the unorthodoxy of his own "solution" in La Monadologie that, like Spinoza in the case of his Ethics, he did not dare to have it published in his life-time. Curiously, Stewart does not mention that. And, describing the pathos of Leibniz's last years, Stewart suggests that at the end of his life he had come to the conclusion that this was not, after all, the best of all possible worlds.
When I was thrown out of my Philosophy course at Warwick University, my defiant riposte to my tutors was that philosophy should be taught in its historical context, not merely as an unmotivated collection of systems of ideas. This was interpreted at the time as juvenile Marxism, but Matthew Stewart brilliantly illustrates how philosophy only makes sense when construed as the systems created by brilliant individuals to make sense of the great issues of their day.
In the case of Spinoza and Leibniz, Stewart shows how those great issues - faith and transcendence vs. the significance of existence within (secular) modernism - have defined the terms of debate from the 17th Century through to the present day.
This is a book on many levels. Most immediately, in interwoven chapters, we are told the lives of these two extraordinary individuals: birth through death. Both Spinoza and Leibniz were observing and conceptually shaping the Age of Reason, but from very different social positions.
From these historical and intellectual foundations, Stewart explains with the greatest clarity the philosophies of the two men: Spinoza's Ethics, and Leibniz's Monadology. If only it had been explained like this at University!
Finally, Stewart critically situates these two systems as the purest opposed responses to post-medieval modernisation, one welcoming it, the other seeking to maintain the place of faith and spirit over a putative reduction of everything to the mundane. These remarks do not begin to do justice to the sophistication of Stewart's analysis. I could add that his witty and irreverent style makes it a joy to read, but perhaps this book has been praised enough!