In his work Religion within The Limits of Reason Alone (1793), Kant founds all ethical theories – and not only his own – on three postulates or necessary, a priori conditions. The latter are not so much presented as theoretical dogmas as ‘reasonable’ presuppositions having a direct impact on our moral choices and decisions.
The first postulate is God whose existence is not revealed through religious experience but lies in our own reason. Abandoning his Pietist upbringing, Kant wants to avoid any philosophical temptation to blindly obey some divine command instead of listening to the voice of our conscience. This awkward need of God arises because the relationship between moral law and happiness is not guaranteed in this world. By introducing God as an idea, as opposed to a reality, Kant points towards the necessary compatibility between virtue as a motivating ideal and the practical realisation of the highest possible moral good.
The postulate of immortality is very much interwoven with the postulate of God. Taking into account the sensuous nature of human beings, Kant states that it is very difficult for a man to be righteous without hope. Immortality guarantees this hope and ensures that there is a place sufficient for the reckoning of happiness in proportion to worthiness to be happy. However, Kant does not mention potential afterlife rewards such as Heaven to ‘good’ moral agents. Instead, he counts on the prospect of possible afterlife moral actions as an incentive to pursue noble aims in this life.
Finally, the postulate of freedom is given a special position among the other two postulates. Freedom is an a priori that we cannot fully grasp but we know it as the condition of the moral law which is within us. It is because of freedom that God and Immortality gain objective reality and legitimacy and subjective necessity. The ideas of God and immortality are not conditions of the moral law, but only conditions of the necessary object of a will which is determined by this law. In other words, both ‘ideas’ of God and immortality give an a priori (non-verifiable) purpose to the moral will which we discover within our pure (abstract as opposed to practical) reason. Kant says in the preface to the Critique of Practical Reason (1788) that the concept of freedom is ‘the key stone of the whole architecture of the system of pure reason and even speculative reason.’
In other words, the moral law is discovered through reason and then put to good use through our moral will as the very expression of our freedom … God and (moral) immortality being the (theological) carrot, as it were.