Not just because we can, although that on its own makes it certain that we will, but because evolution hasn't finished with us, and never will, while we exist; because we seem unable to secure the good bits of human nature against the bad bits; because we may destroy ourselves if we don't change; and because we have allowed the Church and the State to relieve our 'souls' – for want of a better word – of moral responsibility.
Many people will argue against interference with the work – variously – of God, evolution and society; but this argument has no clothes any longer, if it ever did. We have comprehensively re-engineered plants and such species as wolves, cats and horses. We go to enormous lengths to educate children to defy their natures. We have an vast and growing tower of ethical rules which constrain humans to behave in prescribed ways in almost every imaginable situation in life – and, very importantly, these rules are increasingly global, as Book One has shown. It isn't possible any longer to 'let 1,000 flowers bloom' – humanity has reached consensus on an ever wider range of behavioural and economic rules. A process which we have characterized as ineluctable.
These masses of rules are nothing less than an attempt to change human nature by barricading it into a behavioural corner – how much more elegant it would be to change ourselves.