R. Yitzchak says: Even a man who intends to smite one and smites another is not liable — until he makes it clear that it is this man that he wishes to smite, as it is written (Devarim 19:11) "and he rise up against him and he smite him, etc." What, then, is the intent of "and if men fight, etc."? From (Leviticus 24:17) "And if a man smite any soul of man" I would understand even an eight-month birth, (who is destined to die); it is, therefore, written ("and if men fight," to indicate that) he is not liable unless he kills one who is (otherwise) destined to live.