The two faculties we've just examined – external perception and consciousness – yield experience. But can experience alone explain all of our knowledge? Or do other faculties also play a role? We'll consider this now.

To this end, let's determine the characteristics of the judgments we owe to experience. If we then discover judgments that have entirely different characteristics, we'll conclude that another faculty exists within us.

The characteristic of judgments owed to experience is that they're contingent – the mind can conceive of the opposite judgment being made.

Let's take an example from external perception. It's almost universally recognized that bodies fall in a vertical line. Yet it's not hard to imagine that they might well fall in another direction. Epicurus, for example, believed that atoms follow an oblique path. So this judgment is contingent.

Let's take another example. I say: “Man is a being with sensory abilities.” This is true, but we can certainly conceive of a being who would have all the other faculties of man while lacking this one. This judgment is also contingent.

In fact, all judgments derived from experience are contingent. How could it be otherwise? What would keep us from conceiving of the opposite judgment? Certainly not the nature of the mind, from which judgments formed under the influence of the facts remain independent.

But now consider another truth: “Every phenomenon has a cause.” In this case, the opposite judgment is inconceivable. The judgment is thus called necessary.