This article argues that Kant's essay on enlightenment responds to Moses Mendelssohn's defense of the freedom of conscience in Jerusalem. While Mendelssohn holds that the freedom of conscience as an inalienable right, Kant argues that the use of one's reason may be constrained by oaths. Kant calls such a constrained use of reason the private use of reason. While he also defends the unconditional freedom of the public use of reason, Kant believes that one makes oneself a part of the machinery of the church or state by swearing an oath to and assuming a position within those institutions.