The sixth manuscript of Bonhoeffer's Ethics is entitled Natural Life. Bonhoeffer sees this as a neglected subject in Protestant theology (although it features prominently in Roman Catholic theology, where the relationship between nature and grace is understood in a way which is problematic for Protestant thought). Natural life is not the same as created life - rather, "through the fall, 'creation' became 'nature'. The unmediated relation to God of the true creation becomes the relative freedom of natural life" (173). In terms of the relationship between ultimate and penultimate things, natural life is penultimate, and indeed the concept of natural life helps us to distinguish between those things in life which are genuinely penultimate and those which are unnatural and therefore destructive of the penultimate. Bonhoeffer is here establishing a differentiation within the sphere of the world and the penultimate: against those who deride the concept of natural life because they have lumped everything that is not grace together as 'sin', he sees some things as genuinely natural and others as genuinely unnatural. But against the Roman concept which tends to make the natural an 'ultimate' in itself, Bonhoeffer is clear that the natural-ness or unnatural-ness of anything will only be revealed in the light of Christ - when things are seen to be either penultimate preparations of the way of the Lord, or obstructions to his path.
"How is the natural recognized? The natural is that form of life preserved by God for the fallen world that is directed toward justification, salvation, and renewal through Christ" (174). It can therefore only be known "by looking at Jesus Christ" (174). Nevertheless, it is reason which is the human instrument for discerning the natural. The natural, as universally human, is not something which can be defined by any human authority, which will only show itself as arbitrary in so far as it attempts this. Bonhoeffer, of course, has the Nazi state in view.
"The natural guards life against the unnatural" (176). Because the natural is the form of life preserved by God, it fights off the unnatural, which is in itself "the enemy of life" (176). The unnatural may triumph over the natural in limited ways, but the natural will always reassert itself at length. This is because "the unnatural is something that requires organization, while the natural cannot be organized but is simply there" (177). It takes fighting and campaigning and legislation to suppress the natural - I am reminded of various campaigns and organisations in the early twenty-first century. If Bonhoeffer is right, and I think he is, those campaigns are right to think that they need to keep on even after they have apparently achieved their objectives; the natural will in time fight back!
The natural life is a formed life, and its form takes shape in rights and duties, corresponding to the fact that for Christian ethics life is both an end in itself and a means to an end. Rights come first: "that means speaking first of what is given to life, and only then of what is demanded from it" (180). This is the only way in which God is honoured, and it is also logical: "duties spring from the rights themselves, as tasks from gifts" (180). Therefore for the remainder of the manuscript Bonhoeffer expounds particular rights of natural life, including 'to each his own', the inviolability of bodily life, and the rights of reproduction and developing life.
In this connection, and with relevance to contemporary debate, here is another snippet on abortion: "To kill the fruit in the mother's womb is to injure the right to life which God has bestowed on the developing life. Discussion of the question whether a human being is already present confuses the simple fact that, in any case, God wills to create a human being and that the life of this developing human being has been deliberately taken. And this is nothing but murder" (206, emphasis added). Of course there are circumstances (and "without doubt, all this decisively affects one's personal, pastoral attitude toward the person concerned" 181), but they can't change the facts.
I see this manuscript as pursuing one of the most essential insights of Bonhoeffer's work, which is that there are ethical distinctions in the non-Christian world. This might seem obvious, but the temptation of theologians to write off the whole structure as sinful is powerful. Of course, the structure is implicated in sin - we are talking about natural life, not the pristine life of the first creation. But there is nevertheless relative good and relative evil, and the church needs to engage with that if it is to prepare the way for Christ and his gospel in the world.