The preface to Part III begins with a complaint. Writers on the emotions and human conduct often seem to discuss something outside nature rather than something following its common laws. They imagine humanity as a dominion within a dominion.

They suppose that people disturb nature’s order instead of following it, and possess absolute power over their actions. Human weakness and inconsistency are then blamed on a defect of human nature rather than explained through nature’s general power. Such writers lament, mock, despise or detest our failings. Those who denounce them most eloquently may even seem almost divine.

Spinoza acknowledges that excellent writers have offered valuable advice about living rightly. We owe much to their labour. What has not, in his judgment, been adequately determined is the nature and power of the affects and the extent of the mind’s ability to moderate them.

He mentions Descartes, who tried to explain the affects through their first causes while assigning the mind absolute authority over them. Spinoza credits his ingenuity but does not accept the result. He promises to demonstrate the difficulty in its proper place. For now he returns to those who prefer to hate or ridicule the affects rather than understand them.

What will surprise those readers

Those readers, he says, will find his undertaking strange. He proposes to treat human faults and follies geometrically, using certain reasoning to explain things they call contrary to reason, empty, absurd or dreadful.

Then the voice becomes explicit: this is my reason. Nothing happens in nature that can be attributed to a defect in nature. Its power and the laws through which things arise and change are everywhere the same. Understanding any kind of thing must therefore proceed through those common laws.

Hatred, anger and envy, considered in themselves, follow the same necessity as other particular things. They have causes through which they can be understood, and properties worth knowing, no less than the properties of things whose contemplation pleases us.

The preface ends with his undertaking: to discuss the affects, and the mind’s power in relation to them, by the method already used for God and the mind. He will consider human actions and appetites as if examining lines, planes or bodies.

The word “defect”

A form of the same Latin word appears three times in this preface. First, other writers attribute human weakness to a defect in human nature. Second, Spinoza names the human defects and follies he proposes to study. Third, his own explanation begins by denying that anything in nature can be attributed to nature’s defect.

He has not stopped using the ordinary vocabulary of faults. What he rejects is its use as an ultimate explanation. Naming the trouble does not tell us what produced it.

In the supermarket

A child is crying in a supermarket. One adult says this is simply what is wrong with the child. Another wonders what time it is, whether the child slept, whether they are hungry.

Both see the same crying. One response has given it a name; the other begins to look for conditions.

The difference is not that the second adult thinks every outburst desirable. Trying to understand a cause is different from praising what happened.

At the end

The preface’s last sentence ends. Next come the definitions of Part III.

From that point, hatred and anger enter a numbered inquiry, beside lines and planes in the author’s declaration of method.