Full Idea
The moral principle 'it is a duty to tell the truth' would, if taken unconditionally and singly, make any society impossible.
Gist of Idea
Unconditional truth-telling makes a society impossible
Source
Benjamin Constant (On Political Reactions [1797], p.124), quoted by Immanuel Kant - On a supposed right to lie p.28
Book Reference
'Deontology', ed/tr. Darwall,Stephen [Blackwell 2003], p.28
A Reaction
He gives the well known example of the murderer at the door asking if your friend is inside. Compare everyone becoming perfectly telepathic. Our society would collapse, but a new society would learn to live with it.
Related Idea
Idea 7 Surely you don't return a borrowed weapon to a mad friend? [Plato]