The propriety of our moral sentiments is never so apt to be corrupted, as when the indulgent and partial spectator is at hand, while the indifferent and impartial one is at a great distance.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“and if, by the simplicity of your unassuming demeanour, you can gain their favour and kindness, you may rest satisfied that you are modest enough, and that your head has been in no respect turned by your good fortune. The propriety of our moral sentiments is never so apt to be corrupted, as when the indulgent and partial spectator is at hand, while the indifferent and impartial one is at a great distance. Of the conduct of one independent nation towards another, neutral nations are the only indifferent and impartial spectators. But they are placed at so great a distance that they are almost quite out of sight.” source