Part I reaches Proposition Thirty-Six and finishes its sequence of demonstrations. Then an appendix begins. The text no longer divides itself into proposition and proof.

Spinoza gathers what he takes himself to have established: God necessarily exists, is unique, exists and acts through the necessity of his own nature, and is the free cause of all things, which are all in God.

He then turns to the reader. Whenever he had the opportunity, he says, he tried to remove prejudices obstructing his demonstrations. Many remain, especially those that make it hard to accept the connections among things in the way he has explained them. He will now bring those prejudices before reason.

One assumption underneath the others

The prejudices depend chiefly on one assumption. People suppose that natural things act for ends as they do, and that God directs everything toward an end. God made the world for human use, and made humans to worship him.

Spinoza does not begin by attacking the conclusion. He announces three tasks: discover why so many people accept the prejudice and why everyone is naturally disposed toward it; show why it is false; then explain how further notions—good and evil, merit and fault, praise and blame, order and confusion, beauty and ugliness—grow from it.

Before asking why an opinion is wrong, he asks why people find it believable.

A different starting point

He explicitly sets a limit. This is not the place to deduce everything from the nature of the human mind. He begins instead from something he expects everyone to acknowledge: people are born ignorant of causes; they desire what benefits them; and they are conscious of that desire.

The explanation unfolds from there. People know what they want without knowing the causes that make them want it, so they imagine themselves free. Because they act for their advantage, they ask for the purposes of things. Once a purpose is supplied, inquiry tends to stop.

They see eyes that enable sight, teeth that chew, plants and animals that feed them, sunlight and a sea full of fish. They infer one or more rulers of nature who prepared these conveniences for human beings.

The effort to prove that nature does nothing useless—that is, nothing without human benefit—ends, Spinoza says, by making nature and the gods look as irrational as people.

Look where matters have arrived, he asks. Alongside the conveniences stand storms, earthquakes and disease. Those must then be explained as divine anger at human offences.

Mathematics offered another standard of truth. It considers the essences and properties of figures rather than their purposes. Without that example, he suggests, the prejudice could have kept truth hidden from people indefinitely.

When he reaches the promised refutation, he refers back to earlier propositions and adds further explanation. Much of the appendix has instead been occupied with how the belief takes hold.

After the car wash

Someone is convinced that it rains the day after every car wash. You show them a month’s weather records. They nod. The next time they wash the car, they still look anxiously at the sky.

Rather than turn another page of the records, you ask what made this association so persuasive in the first place.

That changes the inquiry. Evidence against a belief and an account of its persistence are not the same kind of work.

At the end

Spinoza closes by saying these are the prejudices he intended to identify. If others of the same sort remain, a little reflection should enable readers to correct them.

Thirty-six propositions have been demonstrated. Now the writer turns to see what the person reading is still holding onto.