When we are about to act, the eagerness of passion will seldom allow us to consider what we are doing, with the candour of an indifferent person.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“first, when we are about to act; and secondly, after we have acted. Our views are apt to be very partial in both cases; but they are apt to be most partial when it is of most importance that they should be otherwise. When we are about to act, the eagerness of passion will seldom allow us to consider what we are doing, with the candour of an indifferent person. The violent emotions which at that time agitate us, discolour our views of things, even when we are endeavouring to place ourselves in the situation of another, and to regard the objects that interest us in the light in which they will naturally appear to him.” source