Not by reason, nor ‘revelation’ (he wrote a critique of the Christian mystic, Swedenborg).
However, he did argue – in the 2 – To fully understand Kant’s rather complex argument, one must really understand his whole philosophy, especially his ontology (theory of existence), epistemology (theory of knowledge – which is linked to his ontology), and his ethical theory.
– Another pure concept of the understanding is causality (logical determinism). we can only make sense of the world/universe, by projecting causality (cause & effect) upon it – though in (noumenal) reality, causality does not exist.
– Kant therefore argues that like space & time, causality is an a priori concept – rather than an actual real law.
– This cause cannot lie within nature as it is the cause of nature (our reason and will being part of nature).
– Therefore the cause must be metaphysical (noumenal).– Causality exists in the phenomenal world – the world of empiricism/natural science.But free will exists in the noumenal world and so intentions are not caused by physical/phenomenal things.You may be determined to act socially, biologically, even logically: the belief that every effect in the universe must have been caused by something else (causality).– Kant actually believed in both causality and in free will. He divided reality into two: phenomena (appearances) and noumena (things-in-themselves).– Kant calls this postulation of God a ‘necessary hypothesis’. Knowledge requires that understanding be combined with perception – a metaphysical being cannot be perceived by definition.– Kant continues by identifying the summum bonum with the Christian notion of the Kingdom of God.The harmony itself is not only a logical expectation, but a necessary reward for being moral.– As the summum bonum is a duty as well as a reward, there must be a God who ensures that this harmony (morality & happiness) is attained.– In sum, the summum bonum, which is a result of reason, postulates a necessary harmony between being moral and being happy.This harmony may not be achieved in our lifetime which postulates the immortality of the soul.