I've been trying very hard to work my way through Derek Parfit's two-volume opus On What Matters (Oxford 2011). But, only half way through the first volume, I am at the point of admitting defeat for a couple of reasons. First, I remain unconvinced that it is possible to attain consensus, even among a small set of ethical philosophers, let alone all members of a larger society (not to mention the entire world) on a universally applicable set of fundamental ethical principles. And Parfit's book would be pretty heavy lifting even for someone who believes his project could ultimately succeed.
Some might argue that reading the book is still worthwhile for all the interesting discussions and novel insights about Kant, among other philosophers. I'm happy to concede that point. But my other fundamental problem is that Parfit throws around terms like "rationality" as if cognitive psychologists such as Herbert Simon, Daniel Kahnemann and Amos Tversky never existed. It seems to me that, in this day and age, ethical principles, like economic principles, need to bear at least some relation to human beings, cognitive warts and all, as described by cognitive and brain scientists. An ethics based purely on "good reasons" would seem to have scant applicability to a species with such defective powers of reasoning. Even if Parfit believes that cognitive science is wrong, defective, or simply inapplicable to the derivation of ethical principles from reasons, he at least bears a burden of explaining why. And it cannot be enough to say that people should be more deliberative and reasonable than they in fact are.