Enlightenment is man's release from his self-incurred tutelage. Tutelage is man's inability to make use of his understanding without direction from another. Self-incurred is this tutelage when its cause lies not in lack of reason but in lack of resolution and courage to use it without direction from another. Sapere aude! 'Have courage to use your own reason!'- that is the motto of enlightenment.
An age cannot bind itself and ordain to put the succeeding one into such a condition that it cannot extend its (at best very occasional) knowledge , purify itself of errors, and progress in general enlightenment. That would be a crime against human nature, the proper destination of which lies precisely in this progress and the descendants would be fully justified in rejecting those decrees as having been made in an unwarranted and malicious manner. The touchstone of everything that can be concluded as a law for a people lies in the question whether the people could have imposed such a law on itself.
Skepticism is thus a resting-place for human reason, where it can reflect upon its dogmatic wanderings and make survey of the region in which it finds itself, so that for the future it may be able to choose its path with more certainty. But it is no dwelling-place for permanent settlement. Such can be obtained only through perfect certainty in our knowledge, alike of the objects themselves and of the limits within which all our knowledge of objects is enclosed.
...[M]oral instruction, although containing much that is convincing for the reason, ...accomplishes... little... [because] the teachers themselves have not got their own notions clear, and when they endeavor to make up for this by raking up motives of moral goodness from every quarter, trying to make their physic right strong, they spoil it.