This phrase is used for a cluster of related problems in reasoning. It can be explained in at least three different ways:
- as an error that occurs when ‘the ought’ is derived from the ‘is’. (We have David Hume to thank for this formulation.)
- as an equation of goodness with some natural property such as pleasantness that normally (or perhaps invariably) accompanies it. (This one comes from G.E. Moore.)
- as an assumption that nature and natural things are clearly more valuable than things which aren’t (i.e. things that might be thought of as artificial). (Nobody specific gets credit for this one.) This is sometimes referred to as the “appeal to nature,” which is often treated as a separate fallacy altogether.
These three constructions of the fallacy are closely related to one another, but they are not identical. Each of them involves different conceptual problems.
Is/Ought: In the first instance, the problem arises in the effort to derive a moral principle from an objective reality. This can be seen, for example, when the notion that certain sexual activities may be more dangerous than others is used to advance a moral obligation to refrain from doing them. (People often engage in dangerous activity without triggering a sense of moral transgression.) It can also be seen, for example, in larger philosophical questions such as the notion that one ought to act in such a manner as to promote the greatest good for the greatest number of people or the idea that certain behavioral characteristics have evolutionary advantages can be used to produce specific ideas about right and wrong. There may or may not be reasonable ways of tackling these issues, but at least some of these inferences appear to bring an unjustified value judgement into the equation.
One might even suggest that most of them do.
Natural Property: In the second instance, the problem lies in equating goodness with a more tangible (natural) quality. This effectively enables a speaker to evoke a sense of objective reality when speaking of moral qualities, but of course the problem lies in the connection between the designated natural quality (e.g. good health or physical pleasure) and its alleged moral significance. Whether or not a reasonable case can be established for such a connection is of course an interesting and very complex philosophical question, but it is common rhetorical practice to base this on little to no reasonable justification, hence it’s fallacious status.
Appeal to Nature: Here the problem lies in more practical distinctions such as natural childbirth versus a C-section, relying on natural immunity versus taking a vaccine, using contraceptives, using cosmetics, etc. While a case might be made for the advantages of some of the “natural” options, it is also common for people to assume the natural quality speaks for itself and the artificial option is clearly bad. Whether or not that is really the case is actually an empirical question.
Needless to say, this variation can be reversed. Someone may take artificial or technologically ‘advanced” approaches to life choices to be an obvious improvement over nature in itself. This too would vary from case to case.