Baruch Spinoza's *Ethica, Ordine Geometrico Demonstrata* was published in 1677 by his friends, a few months after his death. Spinoza had refused to publish it during his lifetime; in 1656 the Amsterdam Jewish community had issued against him a *herem* of unprecedented severity, and he had spent the next twenty years grinding lenses and writing.

The form of the *Ethics* is extraordinary. Like Euclid's *Elements*, it proceeds by definitions, axioms, propositions, and demonstrations. Spinoza is convinced that ethics can be built with the rigour of geometry. The conclusion of Book I is staggering: there is only one substance in the universe, with infinitely many attributes, and self-caused — God. Human beings, nature, stones, and thoughts are not separate things but different *modes* of this one substance. God is not the creator of nature; God is nature: *Deus sive Natura*. The phrase was a scandal to the Christian and Jewish theologies of the day.

The second consequence is equally radical. Since we are within the one substance, the apparent 'freedom' of the will is an illusion. If a stone flying through the air thought itself a free agent, our own sense of freedom is no different. For Spinoza true freedom is the grasp of causal necessity — understanding why we feel what we feel rather than being driven by passions whose sources we do not see.

From this an ethics follows. *Conatus* — each thing's striving to persevere in its own being — is the basic engine of every existent. Blessedness is what Spinoza calls the 'third kind of knowledge', in which the rational mind sees itself as part of the infinite context of God/Nature: *amor intellectualis Dei*, the intellectual love of God. For centuries Spinoza was read as the excommunicated atheist; Goethe, Coleridge, and Einstein were drawn to him. Deleuze called him 'the prince of philosophers'. Today ecological thought and the neuroscience of emotion both return to Spinoza, because he was the first to think through what is at stake in giving up dualism, all the way to the end.