In Act and Being Bonhoeffer employs Lutheran theology in critique of philosophical claims about human consciousness from the Enlightenment (esp. Kant) up to his own time, including Heidegger. In doing so he provides a vindication for God self-disclosure against the beliefs expounded by the metaphysics of transcendental philosophy. Undoubtedly Bonhoeffer's most philosophical and abstract work, it is nevertheless crucial for understanding Sanctorum Communio, and, arguably, all of Bonhoeffer's theology. Bonhoeffer approaches human consciousness from the perspective of the Reformation that understands human sin as 'the heart turned in on itself and thus is neither open to the Revelation of God nor to the encounter with the neighbor.'