It is futile to pursue a political purpose by methods that are bound to fail
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

“The best constitution is one which apportions legal rights among prince, nobles, and people in proportion to their real power, for under such a constitution successful revolutions are difficult and therefore stability is possible; but for considerations of stability, it would be wise to give more power to the people. So far as regards ends. But there is also, in politics, the question of means. It is futile to pursue a political purpose by methods that are bound to fail; if the end is held good, we must choose means adequate to its achievement. The question of means can be treated in a purely scientific manner, without regard to the goodness or badness of the ends.” source