The answer to this question can be found in Kant's famous essay "What is Enlightenment?" In it, Kant argues that man is held back from reaching enlightenment by what he calls "self-imposed nonage." ("Nonage" is an old-fashioned word for immaturity).

Kant further defines nonage as an inability to use one's understanding without another's guidance. This is only to be expected in children; children need guidance from their parents and other authority figures. But adults should have the courage to use their own minds without guidance from anyone else. Only in this way will they ever achieve what Kant regards as true enlightenment.

Every human being has the capacity for rational understanding—which is at the heart of enlightenment—but most people are simply too lazy or too cowardly to exercise that capacity. They would much rather let other people do their thinking for them. Such people would include pastors and physicians.

In keeping with the generally anti-clerical tenor of Enlightenment thought, Kant believed that organized religion deliberately kept people in a state of ignorance, the better to secure their continued loyalty. Those with political authority, such as kings and queens, often did the same, using irrational ideas such as the Divine Right of Kings to justify their power.

Kant doesn't underestimate how difficult it will be for most people to break out from their self-imposed immaturity. For many, allowing others to guide their understanding has become almost second nature. This makes it all the more difficult for them to achieve enlightenment, even if they know it's the right thing to do. However, Kant argues that if the common people are given freedom, both intellectual and political, then he is confident—a little too confident, perhaps—that enlightenment is almost certain to follow.