In the third book of the Ethics, Spinoza writes that he intends to consider human emotions "as if the surfaces of lines, planes or solids". Because the emotions are just as natural and as law-governed as all other modes, he suggests, they can be studied with mathematical precision. And this means that human behaviour, so often motivated by emotion, must be completely intelligible and explicable.Spinoza criticises people who, believing "that man rather disturbs than follows the order of nature, that he has absolute power over his actions", tend to adopt a misguidedly moralistic attitude. "They refer the cause of human weakness and inconstancy not to the common forces of universal nature, but to I know not what vice in human nature, which they therefore bewail, deride, despise, or more frequently detest." Spinoza thought that it was more fruitful to understand our emotions and actions than to hate or ridicule them.
According to Spinoza, we understand something fully when we know what causes it, and how. From the perspective of his philosophy this is rather a tall order, since everything is connected, and therefore the causes of any particular phenomenon are highly complex. In fact, understanding something ultimately means knowing the whole of which it is a part – in other words, knowing God. . . .