We are all children of a mindset which has long taken for granted that man's domination of nature — everything from learning to rotate crops to putting a man on the moon — is a good thing.
And the vast majority of it has been good. This drive toward domination has been a dominant passion of the western mind in particular, especially from the 16th century onward, with roots in the thought of Francis Bacon and Rene Descartes. But as the Holy Father points out, the drive toward domination has brought singular perils for humanity. Consider, for example, the drive to harness the forces of atomic nuclei (making nuclear weapons possible), and in our day, the drive to harness the processes of cell regeneration (which is currently ushering in the age of embryo-destructive research).
While it is certainly possible to employ this same knowledge and technical ability for the good of humanity, the Pope is reminding us that the expansiveness of the drive for dominion over nature also holds out the potential for catastrophe. More precisely, his point is that, in many sectors, the drive to dominate nature has now collapsed upon those who would be the dominators, upon human nature itself: "[Man] is now able, so to speak, to 'construct' man on his own." Indeed he is; the age of "designer babies" more than confirms and illustrates this point.
The Pope's concern, then, seems to be this: what happens to our culture when the vast majority of people become accustomed to thinking broadly about human life, human nature, and human existence as objects, as things ‘make-able,' in place of considering them as God-given realities; when they think about the human as a product, as one more potential artifact to be produced by human ingenuity? In Benedict's opinion, what happens is that we descend from understanding ourselves as being created in the image of God, to understanding ourselves as being made in our own image. The Holy Father continues:
All this demonstrates that the growth of our possibilities is not matched by an equal development of our moral energy. Moral strength has not grown in tandem with the development of science; on the contrary, it has diminished, because the technological mentality confines morality to the subjective sphere. Our need, however, is for a public morality, a morality capable of responding to the threats that impose such a burden on the existence of us all. The true gravest danger of the present moment is precisely this imbalance between technological possibilities and moral energy (p. 27, emphasis my own).
It is no secret that persons who espouse what Benedict calls a "technological mentality" will almost by default relegate moral discourse and the whole broad enterprise of ethical consideration to the realm of the utterly subjective. Recourse to that realm — so the idea goes — may have its usefulness (things like religious sentiment and moral consideration can give us a helpful psychological boost from time to time), but it is otherwise utterly unempirical, essentially irrational, and entirely skewed by one's tastes and preferences. It is certainly not the stuff of science, and morality — like religion — is best kept out of the public square.