Can we logically derive a conclusion about how things ought to be from premises only about how things are?
Explain Hume's is-ought gap and Moore's naturalistic fallacy and assess whether moral facts can be reduced to natural facts
Hume noticed that arguments often slide from descriptive premises to a moral conclusion without justification. Moore argued that goodness cannot be defined by any natural property. Together these claims challenge attempts to ground ethics in facts about nature.
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What this dot point is asking
You need to state Hume's gap and Moore's fallacy accurately, distinguish them, and evaluate whether they succeed in showing that moral claims cannot be reduced to factual ones.
Hume's law: the gap between is and ought
In A Treatise of Human Nature, David Hume observed that moral writers proceed for a while with ordinary factual claims joined by is and is not, and then suddenly switch to claims joined by ought and ought not. He pointed out that this new relation needs explaining, because it seems inconceivable that an ought conclusion could be deduced from premises that are entirely about what is. This has become known as the is-ought gap or Hume's law.
The logical point is simple but powerful. A valid deductive argument cannot contain anything in its conclusion that is not already present in its premises. So if every premise is purely descriptive, a prescriptive conclusion cannot follow validly. To get from facts about human nature to a conclusion about how we ought to live, you need at least one premise that is already evaluative, and that premise is exactly what is in dispute.
Moore's naturalistic fallacy
G. E. Moore, in Principia Ethica, attacked a different but related target. He argued that good is a simple, unanalysable property, like the colour yellow, which cannot be defined in terms of anything else. Theories that identify goodness with a natural property, such as the claim that good means pleasant or means what we desire to desire, commit what he called the naturalistic fallacy.
Moore's main tool is the open question argument. Take any proposed definition of good, such as good means pleasant. We can always sensibly ask, but is pleasure actually good? The question stays open and meaningful, which it would not if good simply meant pleasant. Therefore goodness cannot be identical to any natural property, and naturalistic definitions fail.
How the two arguments differ
The two are easy to run together but they are distinct. Hume's point is logical: you cannot validly cross from is to ought. Moore's point is semantic and metaphysical: the word good does not name a natural property at all. You could accept Hume's gap while denying Moore's fallacy, for instance by holding that there are irreducible moral facts that simply are not reachable by deduction from natural ones.
Evaluation: does ethics float free of facts?
These arguments are forceful but not decisive. The open question argument has been criticised for assuming that all genuine identities must be obvious; water just is H2O even though water is H2O is not true by definition, so an identity between goodness and a natural property need not be transparent. This is the line taken by Cornell realists and other defenders of synthetic ethical naturalism.
Against Hume, some argue that certain descriptive facts carry evaluation built in. John Searle claimed that from the fact that someone made a promise we can derive that they ought to keep it, because promising is an institution defined partly by obligation. Whether this really bridges the gap or just relocates the hidden evaluative premise remains debated. The safe conclusion for an exam is that Hume and Moore set a genuine burden of proof on anyone who would ground morality in nature, a burden that naturalists must meet rather than ignore.