Education and the Good Life by Bertrand Russell

To produce the habit of truthfulness should be one of the major aims of moral education. I do not mean truthfulness in speech only, but also in thought; indeed, of the two, the latter seems to me the more important. I prefer a person who lies with full consciousness of what he is doing to a person who first subconsciously deceives himself and then imagines that he is being virtuous and truthful. Indeed, no man who thinks truthfully can believe that it is _always_ wrong to speak untruthfully. Those who hold that a lie is always wrong have to supplement this view by a great deal of casuistry and considerable practice in misleading ambiguities, by means of which they deceive without admitting to themselves that they are lying. Nevertheless, I hold that the occasions when lying is justifiable are few -- much fewer than would be inferred from the practice of high-minded men.

And almost all the occasions which justify lying are occasions where power is being used tyrannically, or where people are engaged in some harmful activity such as war; therefore in a good social system they would be even rarer than they are now.