if each man in fact and inevitably pursues his own pleasure, there is no point in saying he ought to do something else.
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“He does not notice that a thing is “visible” if it can be seen, but “desirable” if it ought to be desired. Thus “desirable” is a word presupposing an ethical theory; we cannot infer what is desirable from what is desired. Again: if each man in fact and inevitably pursues his own pleasure, there is no point in saying he ought to do something else. Kant urged that “you ought” implies “you can”; conversely, if you cannot, it is futile to say you ought. If each man must always pursue his own pleasure, ethics is reduced to prudence: you may do well to further the interests of others in the hope that they in turn will further yours.” source