If Descartes set the agenda for seventeenth-century rationalism, it was his successors who demonstrated what that agenda could produce when followed with full logical consistency to its ultimate conclusions. The result was simultaneously magnificent and alarming.
Baruch Spinoza, reading Descartes with a rigour that its author might have found unsettling, took the concept of substance and followed its implications to a destination that scandalised the age: a single infinite substance identical with God and with Nature, governed entirely by necessity. Gottfried Wilhelm Leibniz, horrified by Spinoza’s conclusions but unwilling to accept the Cartesian problematic that had generated them, proposed an alternative universe of irreducibly mind-like simple substances — monads — each a living mirror of the whole cosmos, coordinated by a pre-established harmony instituted by God.
Between them, they carried rationalism to heights — and into difficulties — that its founder had neither intended nor anticipated.
Spinoza: The Heretic of Amsterdam
Baruch Spinoza was born in Amsterdam in 1632 into a community of Portuguese-Jewish emigrants who had fled the Inquisition. He was the star pupil of the Talmud Torah school, learning Hebrew, Scripture, the Talmud, and the great tradition of Jewish rationalist philosophy. On 27 July 1656, the elders of the Amsterdam Sephardic community pronounced against him the herem — a formal excommunication of unusual severity. The ban was never rescinded, and Spinoza never sought its reversal.
He withdrew from the Jewish community, Latinised his name to Benedictus, took up the trade of lens grinding, and devoted the rest of his life to philosophy. He lived quietly in Rijnsburg, Voorburg, and finally The Hague. In 1670 he published, anonymously, the Theological-Political Treatise, one of the founding documents of modern biblical criticism and liberal political theory. It argued that Scripture should be interpreted by the same historical and philological methods applied to any ancient text, that miracles are not violations of natural law, and that freedom of thought is essential to civil order. The book caused an immediate scandal and was banned.
The work for which he is remembered above all, the Ethics Demonstrated in the Geometrical Manner, was published only after his death in 1677. It is one of the most extraordinary philosophical texts ever written: five books proceeding by numbered propositions, each followed by a proof referencing only earlier elements, in the manner of Euclid.
The One Substance
The metaphysical starting point is the concept of substance, defined as “that which is in itself and conceived through itself” — an entity that requires nothing beyond itself. Descartes had populated the universe with three kinds of substance. Spinoza follows the argument wherever it leads.
If a substance is by definition self-sufficient, then there can be only one substance. One substance, infinite in all its attributes, necessarily and eternally existing: this is what Spinoza calls God, and the identification of God with Nature — Deus sive Natura — is a metaphysical theorem.
Everything else — stones, bodies, minds, ideas, emotions — is a mode: a finite expression of infinite being. There is no Cartesian dualism of mind and matter as separate substances, only two of the infinite attributes under which the single substance can be conceived. Thought and extension are not two different kinds of being; they are two ways of seeing the same being. The order and connection of ideas is exactly the same as the order and connection of things.
Freedom Through Understanding
This determinism is not the counsel of despair. Spinoza draws a distinction between the first and third kinds of knowledge. The first kind is imagination — confused, fragmentary awareness that produces fear, hope, and superstition. The third kind is intuitive science, the direct intellectual grasp of things sub specie aeternitatis — under the aspect of eternity — that sees each thing as a necessary expression of God’s nature.
To understand something as necessary is to be freed from one’s emotional bondage to it. We are enslaved by passions precisely because we conceive of ourselves and the things we desire as contingent. When we understand that everything is as it must be, the passions lose their compulsive grip. The result is the amor Dei intellectualis, the intellectual love of God — not the love of a personal deity who might respond to prayers, but a love that is identical with understanding. It is the highest happiness available to a human being.
Leibniz: Monads and Pre-established Harmony
Gottfried Wilhelm Leibniz was born in Leipzig in 1646. A polymath by instinct, he studied jurisprudence, mathematics, theology, and philosophy. He met the leading mathematicians of his day, conceived the differential calculus (independently of Newton), designed a calculating machine, and spent three days in conversation with Spinoza in 1676. He never held an academic position, serving instead as librarian and court historian to the Dukes of Brunswick in Hanover.
Leibniz identified the central problem in the Cartesian tradition as the concept of extension. Extension is infinitely divisible; there is no natural stopping point. But genuine substances must be real, unified, indivisible beings. The ultimate constituents of reality, Leibniz argued, are not material atoms but simple, immaterial, mind-like substances that he called monads.
Each monad has no spatial extension. What it has is perception — a representation of the entire universe from its own unique perspective — and appetition, an internal principle of activity. Each monad is a living mirror of the whole cosmos.
The problem of how these monads relate is solved by pre-established harmony. God has so programmed each monad that its internal states correspond perfectly with all the others, without any direct causal interaction. The appearance of interaction is just that — appearance.
Theodicy and the Best of All Possible Worlds
Leibniz’s most famous (and most maligned) doctrine is that this is the best of all possible worlds. In his Theodicy (1710), he argued that God, being perfect in power, wisdom, and goodness, must have chosen the best possible world from among the infinite alternatives. The existence of evil does not contradict this claim, because any world containing free moral agents will necessarily contain the possibility of evil, and a world with free agents and some evil may be better, all things considered, than a world without free agents at all.
The claim became the target of Voltaire’s satire in Candide. Yet the underlying intuition — that the universe is rationally ordered and that apparent evils may be necessary components of a greater good — has remained a persistent theme in Western thought.
The Crisis of Rationalism
Between Spinoza’s monism and Leibniz’s pluralism, rationalism had reached a kind of internal crisis. If reason, properly applied, could produce such radically different results, then the confidence that the geometric method could yield a single, indisputable system of truth about God, nature, and the human condition began to look premature.