Hume also put forward the is–ought problem, later called Hume’s Law, denying the possibility of logically deriving what ought to be from what is. He wrote in the Treatise that in every system of morality he has read, the author begins with stating facts about the world, but then suddenly is always referring to what ought to be the case. Hume demands that a reason should be given for inferring what ought to be the case, from what is the case. This because it “seems altogether inconceivable, how this new relation can be a deduction from others”.
To put it another way, there is no rational way to convince me that I ought to do anything. All such efforts must come down to some presupposition or assumption that cannot be rationally compelling.
For example, though it may be true that human beings desire happiness, it does not follow that they ought to desire happiness, or accede to this desire, or that this desire is therefore good.
Nor that it would be bad or wrong to act against this desire, or to desire unhappiness instead.
The fact that I do desire happiness does not rationally compel me to accept that this desire for happiness has normative power.
This argument can be used against any system of ethics. Or indeed, against any normative statement.
The problem with it is that it attempts to rationalise ethics to a point that neglects or misconstrues what ethics actually is.
Ethics is an applied science that arises only because people desire happiness. If we did not desire happiness, then we would not need ethics. If we had no stake in outcomes, we would not need ethics. If we were purely rational observers, we would not need ethics.
That doesn’t mean we would only act rationally…remember, you can’t get an ought from an is, so even a purely rational being would not be able to convince itself that it ought to act rationally.
Reason is neutral. Human beings are not.
That is why I refer to ethics – by which I mean ideally the Natural Law theory of ethics – as an applied science, applied reason, or practical reason.
It is reason applied to the problem of happiness as a desired albeit ambiguous state of being.
Unfortunately, ethics has shifted and changed a great deal over the centuries, and in part thanks to the general effort to make Philosophy and Ethics more like the natural sciences, we’ve ended up with a confused understanding of what ethics is, or was ever meant to be.