When we know the truth, we're sure of it. But sometimes we have the same degree of certainty even when in error. So error is simply false certainty – certainty with no foundation in reality. A theory of error, therefore, should be included in our discussion of certainty.

What's the cause of error?

Spinoza as well as others believed that error is simply partial truth. Errors arise when, seeing but one part of the truth, we mistake it for the whole. Spinoza liked to give the following example: If we think of man in isolation, abstracted from everything else, he seems to be an independent and complete being, free and self-reliant. Yet this is wrong, for we've seen but one part of the truth. We've forgotten that man is part of a larger world from which he can't be extracted. If we'd seen the whole picture, we'd understand the depth of man's dependence. Rather than appearing to be an empire within an empire, man would appear one part of a whole.

But is Spinoza's view correct? Is error simply partial truth? Not at all. For if seeing “less than the truth” leads us to err, why doesn't seeing “more than the truth” have the same effect? The mind is no more “adequate for things” in the one case than in the other. For example, in a vacuum, water rises to ten meters. To explain this, we say that nature abhors a vacuum.