Foucault critiques Kant’s article Was ist Aufklärung? and notes that Kant believes that enlightenment is both and individual task and a responsibility of society. He shows how Kant believes enlightenment cannot be achieved whilst we are immature, by which he means that we simply take orders where reason could be applied. Examples are given. Relying on the answer in a book, relying on the orders of a spiritual advisor for our conscience, and being told by a doctor what to eat.
Kant observes the way reason can be applied in public for reason’s sake. We might pay our taxes and then debate the rationale behind their payment, or we could give pastoral care and then debate religious dogma. Using reason for reason’s sake demonstrates maturity, a necessary step toward achieving enlightenment. He distinguishes between public and private reason and notes that private reason may be willfully circumscribed by reason of a person’s station in life. A person in the army may not have the same degree of latitude to debate an order but reason would have it that obeying the order provides a worthy end.
“…when one is reasoning as a member of reasonable humanity, then the use of reason must be free and public. Enlightenment is thus not merely the process by which individuals would see their own personal freedom of thought guaranteed. There is Enlightenment when the universal, the free, and the public uses of reason are superimposed on one another.”
I don’t think Kant was advocating reason for reason’s sake. He was advocating public reason for the sake of enlightenment. The principle of reason (Everything must have a reason) provides no reason for itself. There is no reason for reason, and so no reason for reason’s sake.