Full Idea
Truthfulness is a duty that must be regarded as the basis of all duties to be grounded on a contract, the law of which is made uncertain and useless if even the least exception to it is admitted.
Gist of Idea
If lies were ever acceptable, with would undermine all duties based on contract
Source
Immanuel Kant (On a supposed right to lie [1797], p.30)
Book Reference
'Deontology', ed/tr. Darwall,Stephen [Blackwell 2003], p.30
A Reaction
Should we keep contracts which are made by means of deception and coercion? Where could such absolute authority for contracts come from? Do contracts and treaties tend to lapse after a long period of time?