Hume’s law states normative statements cannot be logically deduced from descriptive statements. This is the “is-ought problem”.
The is–ought problem, as articulated by the Scottish philosopher and historian David Hume, arises when one makes claims about what ought to be that are based solely on statements about what is. Hume found that there seems to be a significant difference between positive (or descriptive) statements (about what is) and prescriptive or normative statements (about what ought to be), and that it is not obvious how one can coherently transition from descriptive statements to prescriptive ones. Hume’s law or Hume’s guillotine is the thesis that an ethical or judgmental conclusion cannot be inferred from purely descriptive factual statements.
Hume’s idea seems to be that you cannot deduce moral conclusions, featuring moral words such as ‘ought’, from non-moral premises, that is premises from which the moral words are absent. The passage is summed up in the slogan ‘No-Ought-From-Is’ (or NOFI for short) and for many people it represents the take-home message of Hume’s moral philosophy. It is sometimes rather grandly referred to as Hume’s Law.
The is-ought problem has become prominent in matters of ethics and meta-ethics. Simply put, it deals with an apparent logic gap between statements of what “ought” to be, following statements regarding what “is” — the first often following the second without any kind of explanation regarding why they are logical or correct.
Furthermore, it argues that just because someone has knowledge of how the world is (descriptive statements), this doesn’t automatically prove that they know how the world ought to be (prescriptive statements), and it is in fact impossible to derive the second based solely on the information of the first.
A fundamental assumption is the mind must be explained from a materialistic, empiricist position.
He was convinced that the only way to improve philosophy was to make the investigation of human nature central—and empirical (HL 3.2).
Given a naturalistic position, there is no deductive line of reasoning to arrive at an “ought” from an “is”.
Hume famously closes the section of the Treatise that argues against moral rationalism by observing that other systems of moral philosophy, proceeding in the ordinary way of reasoning, at some point make an unremarked transition from premises whose parts are linked only by “is” to conclusions whose parts are linked by “ought” (expressing a new relation) — a deduction that seems to Hume “altogether inconceivable”.
The corollary of the is-ought problem is normative ethics cannot be rationally formulated from a naturalistic position.
According to the dominant twentieth-century interpretation, Hume says here that no ought-judgment may be correctly inferred from a set of premises expressed only in terms of ‘is,’ and the vulgar systems of morality commit this logical fallacy. This is usually thought to mean something much more general: that no ethical or indeed evaluative conclusion whatsoever may be validly inferred from any set of purely factual premises.
Sam Harris is one of the few people who have tried to solve this problem. I addressed his argument in Sam Harris – The Moral Landscape.
Another corollary to the is-ought problem is the naturalistic fallacy.
In philosophical ethics, the naturalistic fallacy is the claim that it is possible to define good in terms of natural entities, or properties such as pleasant or desirable. The term was introduced by British philosopher G. E. Moore in his 1903 book Principia Ethica.
he term naturalistic fallacy is sometimes used to describe the deduction of an ought from an is (the is–ought problem). This usually takes the form of saying that If people do something (e.g., eat three times a day, smoke cigarettes, dress warmly in cold weather), then people ought to do that thing. It becomes a naturalistic fallacy when the is–ought problem (“People eat three times a day, so it is morally good for people to eat three times a day”) is justified by claiming that whatever practice exists is a natural one (“because eating three times a day is pleasant and desirable”).
The inverse of deriving an “is” from an “ought” is the moralistic fallacy.
The moralistic fallacy is the informal fallacy of assuming that an aspect of nature which has socially unpleasant consequences cannot exist. Its typical form is “if X were true, then Z would happen! Thus, X is false”, where Z is a morally, socially or politically undesirable thing. What should be moral is assumed a priori to also be naturally occurring. The moralistic fallacy is sometimes presented as the inverse of the naturalistic fallacy.
Steven Pinker writes that “[t]he naturalistic fallacy is the idea that what is found in nature is good. It was the basis for social Darwinism, the belief that helping the poor and sick would get in the way of evolution, which depends on the survival of the fittest. Today, biologists denounce the naturalistic fallacy because they want to describe the natural world honestly, without people deriving morals about how we ought to behave (as in: If birds and beasts engage in adultery, infanticide, cannibalism, it must be OK).” Pinker goes on to explain that “[t]he moralistic fallacy is that what is good is found in nature. It lies behind the bad science in nature-documentary voiceovers: lions are mercy-killers of the weak and sick, mice feel no pain when cats eat them, dung beetles recycle dung to benefit the ecosystem and so on. It also lies behind the romantic belief that humans cannot harbor desires to kill, rape, lie, or steal because that would be too depressing or reactionary.”
These fallacies support the idea that science cannot explain human morality. Science can explain how the world “is”. But it cannot explain how the world “ought”.
Natural science can help humans understand the natural world, but it cannot make policy, moral, or behavioral decisions.
The is-ought problem also leads to non-cognitivism and it’s impossible to have any claim of moral knowledge.
Non-cognitivism is the meta-ethical view that ethical sentences do not express propositions (i.e., statements) and thus cannot be true or false (they are not truth-apt). A noncognitivist denies the cognitivist claim that “moral judgments are capable of being objectively true, because they describe some feature of the world”. If moral statements cannot be true, and if one cannot know something that is not true, noncognitivism implies that moral knowledge is impossible.