Spinoza had probably arrived at his broad philosophical position before his intensive study of Descartes’ Principles of Philosophy. Nevertheless, his exposition of his system in the Ethics is heavily indebted to Cartesian concepts and terminology, and to his engagement with the problems Descartes’ philosophy gave rise to. Since you have read the relevant sections of Descartes’ Principles, I shall base this account of Spinoza on his reaction to Descartes, even though he does not himself do so explicitly. Spinoza regarded his philosophy as timelessly true (like Euclid’s geometry), and he would have rejected any attempt to put it into a historical context.

For a start, Spinoza held that Descartes was wrong to ground his whole system on his subjective perception of his own existence, and only then to move on to a proof of God’s existence. If God is the source of all reality and all truth, his existence cannot depend on Descartes’ cogito argument, and God must come right at the beginning. Assuming the ontological argument is valid, it is entirely self-standing, and nothing else need come before. To understand the correct definition or concept of God is to understand that he exists. But, as we shall see, what Spinoza means by ‘God’ is hardly different from something like ‘being in general’ — and it is not wholly implausible to claim that it is a necessary truth that ‘being is.’ Spinoza is very reminiscent of the Greek philosopher Parmenides, who derived everything from the proposition: ‘It is.’

In Spinoza’s view, Descartes came close to a correct understanding of God when he said that, strictly speaking, he was the only substance, since he was the only being which was entirely self-subsistent (or ‘cause of itself’ in Spinoza’s terminology). But for Spinoza, we must always speak strictly, and it isn’t good enough to say that ‘extended thing’ and ‘thinking thing’ are substances in some secondary sense. They are not substances at all, if substance is defined in terms of its self-subsistence.

Consequently, it makes no sense to say that God created two substances (or kinds of substance) characterised by the attributes of thought and extension, which are somehow existentially separate from him, even if still dependent on him for their continued existence. Instead we have to say that thought and extension are attributes of God himself.

Another way of looking at it (though I can’t be certain whether Spinoza actually looked at this way) is that we can have no conception of substance as distinct from its attributes. If matter = substance + extension, and mind = substance + thought, and God = substance + whatever attributes he has, then what is the difference between these three ‘substances’? What possible grounds could we have for saying that there is more than one? Consequently there is just one substance, which is understood through its attributes alone.

We have seen how Descartes (usually at least) held that there is really only one extended substance, and that individual material objects are only transitory modes of this one substance, like waves in the ocean. But why treat thinking substance differently? Descartes thought that he could intuit his own real individuality while reflecting on his existence — but this was mere wishful thinking. Minds are in the same position as bodies. Bodies are modes of God’s attribute of extension, and minds are modes of God’s attribute of thought.

Again, Descartes limited himself to two kinds of created substance. But if they are attributes of God, God could not be infinite if he had only two attributes. So he must have infinitely many attributes, even if we mortals can conceive only two of them.

Clearly, this God is not the Great Daddy in the Sky of conventional religion. He is the whole of reality or being in all its aspects, and must not be conceptualised in anthropomorphic terms. This is why Spinoza often referred to him as ‘God or Nature’ (note that in Latin there is no problem of whether to refer to God as ‘he’, ‘she’, or ‘it’, since the distinction is grammatical rather than sexual). He (or she, or it) does not understand, or will, or perceive, or feel. Whatever happens just emanates inexorably from his nature.

Descartes’ fond belief that he had freedom of the will, however inexplicably, was another illusion. It was merely due to his ignorance of the causes of his actions. Everything is in God, and follows from his nature with the same necessity as conclusions follow from premises.

Nor does God have freedom of the will, since that implies that he could choose between alternative possibilities. But since everything emanates by necessity from God’s nature, the only possible universe is the actual one. God would not be perfect if anything conceivable were left unactualised; therefore anything unactualised is inconceivable.

Descartes held that abstract and moral truths are true because of God’s arbitrary decision. For example, God decided to create a world in which the angles of a triangle added up to 180° rather than some other number, and in which killing was an evil rather than a noble deed; and such truths seem obvious to us because God planted innate ideas of them in our minds. Descartes came to this view because he didn’t want God’s powers to be limited by any standard of truth or goodness external to himself. For Spinoza, this struck at the very roots of rationality. If geometrical truths were arbitrary, and only seemed obvious because we had been mesmerised by God, then there could be no rational understanding of anything. Rather, these truths belong to and emanate from God’s rational nature; and since nothing else exists, there can be no question of God’s being subordinated to external standards.

Then there was Descartes’ mess over the dualism of mind and body. It was completely absurd of him to attempt to explain why the one might cause changes in the other, since mind and body are utterly distinct categories. But Descartes’ alternative account (that the union of mind and body is a special category of thing which we cannot understand) is no explanation at all — it is merely to give up. For Spinoza, mind and body were different aspects of one and the same substance and its modes, and there could be no question of any interaction.

For every body, or mode of extension, there is a corresponding idea, and vice versa. Corresponding to a chair, there is the idea of the chair; and corresponding to a human body there is the idea of that body; and the idea of the body is the mind or soul. Just as the attribute of extension is the totality of extended things, so the attribute of thought is the totality of ideas. Therefore mind and body are not distinct entities, but different ways of considering one and the same thing — as having certain physical characteristics, or as having certain ideas.

As well as ideas, there are ideas of ideas, and ideas of ideas ad infinitum (compare Hobbes’s names of names, or ‘names of the second intension’). These account for human self-consciousness: we do not merely have an idea of our body, but we have an idea of that idea.

Nor is the soul a simply unity, as Descartes maintained, but it is as infinitely complex as the body, since for every part of the body there is a corresponding idea. This explains how it is possible for us to be aware of many things at one and the same time.

In modern terms, one might say that there is a single series of events, which can be considered either as brain events (opening up the skull and observing what happens with probes and microscopes), or as ideas (which is how we are aware of them internally).

Since bodily and mental events are one and the same thing, there is no question of the one causing the other. Nevertheless, they are all caused, since the universe is a deterministic system. Brain events can be understood causally as part of an evolving mechanistic system; and ideas can be understood causally as part of an evolving logical system. The two types of explanation must never be mixed, since that would be a category confusion. Nevertheless, they are equivalent to each other, since they are only different aspects of one and the same system. Ultimately there is no distinction between a causal connection and a logical connection: ‘cause’ and ‘reason’ have the same meaning, and the following of an effect from a cause is the same as the following of a conclusion from a premise.

Descartes had the problem of deciding whether sensations were modes of matter or of mind, and he never came to a clear decision on the issue. On the one hand, they seem to be part of our stream of consciousness; on the other hand, they are extended, which means that they must belong to the brain. For Spinoza, there was no such problem, since a single thing could be both extended and thinking. However, Spinoza still had to account for the difference between sensation, and distinctively human thought (and indeed, for the difference between the human soul, or idea of the body, and the idea of a chair).

Spinoza’s solution was to distinguish between three kinds of knowledge:

Every individual thing has a conation (the same term as in Hobbes), or striving to persist in its own being or essence. In the case of physical objects, it is manifest in resistance to penetration (solidity) or change of motion (inertia). In animate things, it is an ‘appetite’ or motivation towards self-preservation and self-perfection. Since pleasure is (by definition) perception of a transition to greater perfection, and pain its opposite, this is equivalent to saying that we are motivated to pursue pleasure, and to avoid pain. Again, the good is (by definition) what we desire, and the evil what we avoid; but since we desire and avoid different things, the terms ‘good’ and ‘evil’ have no objective or absolute meaning.

However, it is important to distinguish between two types of emotion. Emotions which arise from the senses are ‘passive’, since they happen to us from external causes. But there are other emotions which derive from the understanding, and are ‘active’, in that they are internally generated. These cannot be painful, since pain is something which diminishes rather than enhances the power of the understanding. In so far as we are dominated by passive emotions, we are in a state of servitude; but in so far as we are dominated by active emotions, we are in a state of freedom and unalloyed pleasure. It is within our power to consider things under the attribute of thought rather than under the attribute of extension (i.e. to replace a passive emotion by an active one). Instead of getting upset about unpleasant external influence over which we have no control (as most people do), the philosopher has the intense pleasure of seeing everything as an essential component of the most perfect universe. Ultimately, the highest human freedom and self-fulfilment is to be found in intellectual contemplation. And since the highest level of knowledge is the vision of all things in God, the pleasure it gives is the ‘intellectual love of God.’ (This ethical theory is almost pure Stoicism; and it suffers from the same problem of reconciling the relative freedom of the intellect with an absolute determinism.)

Finally, Descartes believed he had discovered a proof of the natural immortality of the soul. Spinoza’s philosophy has no room for immortality — less even than Hobbes’s. The only sense in which the soul can be said to be immortal is that the idea of the body is eternal, as one component of the infinite system of ideas which constitutes God’s attribute of thought. But this is an immortality without life or consciousness. For Spinoza, we have only one brief life (in his case, all too brief), in which to overcome demeaning, passive emotions, and to achieve as deep an intellectual vision as possible of all things in God (or Nature).

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