Enlightenment is man's emergence from his self-imposed immaturity.
Immaturity is the inability to use one's understanding without guidance from another.
This immaturity is self-imposed when its cause lies not in lack of understanding, but in lack of resolve and courage to use it without guidance from another. It is so easy to be immature. If I have a book to serve as my understanding, a pastor to serve as my conscience, a physician to determine my diet for me, and so on, I need not exert myself at all. Having first made their domestic livestock dumb, and having carefully made sure that these docile creatures will not take a single step without the go-cart to which they are harnessed, these guardians then show them the danger that threatens them, should they attempt to walk alone. Thus, it is difficult for any individual man to work himself out of the immaturity that has all but become his nature.