One of the more striking incoherences characteristic of the Enlightenment is the struggle to establish a naturalistic foundation for morality and ethics. Given the success of the natural sciences and overall confidence in the human cognitive capacities to understand and explain nature in a purely mechanistic way, the hopes of Enlightenment thinkers must have been high to find a new, objective basis for the universal moral law. However, as David Hume noted, there is a logical gap between describing nature (the “is”) and prescribing actions (the “ought”):
“David Hume famously exposes the fallacy of deriving a prescriptive statement (that one ought to perform some action) from a description of how things stand in relation to each other in nature.” (section 2.2. of this SEP article)
The gap occurs due to something we could call an explanatory imbalance. On one side, in its characteristic revolt against any authority, Enlightenment gets rid of any sort of formal or final causes when explaining nature and its mechanisms (these cannot be established by using naturalist methods, they are not needed to explain how natural processes work, and should not be accepted on authority alone). Natural processes are understood as mechanisms without any inherent objectives. On the other side, there is human thought and actions, which cannot be successfully explained by reduction to mere mechanistic processes. There appears to be a teleological aspect to humans. This is where explanatory imbalance becomes visible.
Firstly, it is problematic to ground teleological human thought and actions in a natural order that is purely mechanistic. This is how I understand Hume’s remark that we cannot derive an “ought” from a merely descriptive “is”. Secondly, there appears to be a deeper tension here. If nature can be successfully explained in mechanistic terms and human beings are just a part of nature (as opposed to, say, occupying a special place in creation as being made in the image of God), then what is the basis of assuming that there is anything like a “highest end for human beings” (2.2.)? I see this as one of the biggest challenges inherited from the Enlightenment that still endures (e.g., discussions about free will, moral objectivism/relativism/intersubjectivism etc.) and that has likely served as fertile ground for the development of such philosophical movements as existentialism, absurdism, nihilism and others.