When a man says “this is good in itself,” he seems to be making a statement, just as much as if he had said “this is square” or “this is sweet.” I believe this to be a mistake. I think that what the man really means is: “I wish everybody to desire this,” or rather “Would that everybody desired this.” If what he ways is interpreted as a statement , it is merely an affirmation of his own personal wish; if, on the other hand, it is interpreted in a general way, it states nothing, but merely desires something. The wish, as an occurrence, is personal, but what it desires is universal. It is, I think, this curious interlocking of the particular and the universal which has caused so much confusion in ethics.
The matter may perhaps become clearer by contrasting an ethical sentence with one which makes a statement. If I say “all Chinese are Buddhists,” I can be refuted by the production of a Chinese Christian or Mohammedan. If I say “I believe that all Chinese are Buddhists,” I cannot be refuted by any evidence from China, but only by evidence that I do not believe what I say; for what I am asserting is only something about my own state of mind. If, now, a philosopher says “Beauty is good,” I may interpret him as meaning either “Would that everybody loved the beautiful” (which corresponds to “all Chinese are Buddhists”) or “I wish that everybody loved the beautiful” (which corresponds to “I believe that all Chinese are Buddhists”). The first of these makes no assertion, but expresses a wish; since it affirms nothing, it is logically impossible that there should be evidence for or against it, or for it to possess either truth or falsehood. The second sentence, instead of being merely optative, does make a statement, but it is one about the philosopher’s state of mind, and it could only be refuted by evidence that he does not have the wish that he says he has. This second sentence does not belong to ethics, but to psychology or biography. The first sentence, which does belong to ethics, expresses a desire for something, but asserts nothing.
Ethics, if the above analysis is correct, contains no statements, whether true or false, but consists of desires of a certain general kind, namely such as are concerned with the desires of mankind in general – and of gods, angels, and devils, if they exist. Science can discuss the causes of desires, and the means for realizing them, but it cannot contain any genuinely ethical sentences, because it is concerned with what is true or false.
From Science and Ethics By Bertrand Russell, In Religion and Science (Oxford University Press, 1961)
Via a comment and link from Wes George at ‘Ecology and Ethics (Part 1)’