2. When there is such a mistake in the motive, that had the truth been known, the contract would pot have been made, it is generally void., For example, if a man should, after the death of Titius, of which he was ignorant, insure his life, the error of the motive would avoid the contract. Toull. Dr. Civ. Fr. liv. 3, c. 2, art. 1. Or, if Titius should sell to Livius his horse, which both parties supposed to be living at some distance from the place where the contract was made, when in fact, the horse was then dead, the contract would be void. Poth. Vente, n. 4; 2 Kent, Com. 367. When the contract is entered into under circumstances of clear mistake or surprise, it will not be enforced. See the following authorities on this subject. 1 Russ. & M. 527; 1 Ves. jr. 221; 4 Price, 135; 1 Ves. jr. 210; Atkinson on Titl. 144. Vide Cause; Consideration.
3. The motive of prosecutions is frequently an object of inquiry, particularly when the prosecutor is a witness, and in his case, as that of any other witness, when the motion is ascertained to be bad, as a desire of revenge for a real or supposed injury, the credibility of the witness will be much weakened, though this will not alone render him incompetent. See Evidence; Witness.