For those of you studying IB Philosophy, or taking Higher but interested in going beyond the simplicities of the course, John Nolt’s Environmental Ethics for the Long Term has an excellent section on philosophical arguments in ethics.
Section 2.2.1 has one of the best explanations of the “is/ought” fallacy I have ever read. Using the terms ‘prescriptive’ and ‘descriptive’ to refer to premises that respectively contain or do not contain a sentiment of something being right or wrong, he uses the phrase ‘prescriptive reasoning’ to refer to an argument where (at least) one premise and the conclusion include some sort of moral valuing.
Of course an argument can be valid and sound if it contains no moral sentiments (1), and one which has ‘moral’ or ‘ought’ premises might lead to a valid and ‘ought’ type conclusion (2).
all volvo cars have a steering wheel
my car is a volvo
therefore my car has a steering wheel
one should intervene when one person is abusing another against the latter’s will
‘abusing another against their will’ is what happens in sex-trafficking
therefore you should be acting against sex-trafficking
Of course one might object to the ‘truth’ of each of the premises here, but if one did agree with both then it would commit you to the conclusion. This is an example of what Nolt calls ‘prescriptive reasoning’. The problem is when someone tries to move from purely ‘descriptive’ premises to a prescriptive conclusion. This is the is/ought fallacy. The example used by Nolt is the use of Social Darwinism by the Nazis, simply because the ‘strong’ or ‘fittest’ survive it certainly does not follow that one should act in a way to make this happen.
Allowing this ‘prescriptive reasoning’ to stand is not the same as believing it to be sound. In fact it can be particularly difficult to spot problematic premises. Consider the following:
We ought to eliminate suffering.
The only feasible way to eliminate suffering is to eliminate all sentient beings.
Therefore we ought to eliminate all sentient beings.*
*Nolt, J Environmental Ethics for the Long Term (Abingdon: Routledge, 2015) 39.