The only possible evidence we can acquire for something to be desirable is that it is desired.
In John Stuart Mill’s famous proof of the utility principle, he states that the only evidence that something is desirable is that it is actually desired by someone. Mill introduces this as a methodological premise - whenever somebody or something desires something, we know that this something is desirable.
Well – this is obviously false, right? I can desire something, but I wouldn’t dare to say that it is actually worthy of being desirable, right? For example, I can despise my natural desires and wouldn’t say that they are desirable.
To understand this claim, it’s important to consider where Mill is coming from. The chapter where this idea stems from (the fourth chapter) isn’t meant to prove the utilitarian doctrine. Rather, Mill raises the question what kind of proof one can even present for a final principle. Mill explicitly says that it’s not possible to provide a proof of a last principle. So he rather tries to make it as plausible as possible.
If he tried to prove the utility principle, he would have to show that it follows from some major premise. But this premise would have to be proven as well. Whenever somebody states a last principle, e.g. that we have to judge every act by its tendency to promote wealth, we could ask why wealth is good. But somewhere we have to stop asking. Mill says that this point is marked by the tendency to promote happiness, e.g. that promoting wealth is good because it increases our possibility to do cool things, and cool things are good because they lead to happiness.
So, to save Mill from committing intellectual mischiefs, we can say that by stating that ‘the only possible evidence we can acquire for something to be desirable is that it is desired.’ he meant that the only plausible evidence, the only evidence we should look for when looking for a last principle, is what people actually desire. Mill bases his hedonism and moral theory in human nature, rather than from some abstract speculation.
Source: Mill, J.S. Utilitarianism / Der Utilitarismus. Available at: https://www.reclam.de/detail/978-3-15-018461-5/Mill__John_Stuart/Utilitarianism___Der_Utilitarismus (Accessed: 9 February 2023).