The naturalistic fallacy is an informal logical fallacy which argues that if something is ‘natural’ it must be good. It is closely related to the is/ought fallacy – when someone tries to infer what ‘ought’ to be done from what ‘is’.

The is/ought fallacy is when statements of fact (or ‘is’) jump to statements of value (or ‘ought’), without explanation. First discussed by Scottish philosopher, David Hume, he observed a range of different arguments where writers would be using the terms ‘is’ and ‘is not’ and suddenly, start saying ‘ought’ and ‘ought not’.

For Hume, it was inconceivable that philosophers could jump from ‘is’ to ‘ought’ without showing how the two concepts were connected. What were their justifications?

If this seems weird, consider the following example where someone might say:

  1. It is true that smoking is harmful to your health.
  2. Therefore, you ought not to smoke.

The claim that you ‘ought’ not to smoke is not just saying it would be unhealthy for you to smoke. It says it would be unethical. Why? Lots of ‘unhealthy’ things are perfectly ethical. The assumption that facts lead us directly to value claims is what makes the is/ought argument a fallacy.

As it is, the argument above is unsound – much more is needed. Hume thought no matter what you add to the argument, it would be impossible to make the leap from ‘is’ to ‘ought’ because ‘is’ is based on evidence (facts) and ‘ought’ is always a matter of reason (at best) and opinion or prejudice (at worst).

Later, another philosopher named G.E. Moore coined the term naturalistic fallacy. He said arguments that used nature, or natural terms like ‘pleasant’, ‘satisfying’ or ‘healthy’ to make ethical claims, were unsound.

The naturalistic fallacy looks like this:

  1. Breastfeeding is the natural way to feed children.
  2. Therefore, mothers ought to breastfeed their children and ought not to use baby formula (because it is unnatural).

This is a fallacy. We act against nature all the time – with vaccinations, electricity, medicine – many of which are ethical. Lots of things that are natural are good, but not all unnatural things are unethical. This is what the naturalistic fallacy argues.

Philosophers still debate this issue. For example, G. E. Moore believed in moral realism – that some things are objectively ‘good’ or ‘bad’, ‘right’ or ‘wrong’. This suggests there might be ‘ethical facts’ from which we can make value claims and which are different from ordinary facts. But that’s a whole new topic of discussion.