A vacant point that is immediately adjacent to a stone either directly up, down, left or right from it, or connected through a continuous string of same-colored stones to such a point. A stone, chain or group must always have at least one liberty to survive and a group should usually have at least two separate guaranteed internal liberties (eyes) to be considered alive. If a group is surrounded, having at least two eyes ensures that it cannot be captured by the opponent if correctly defended. Some living groups cannot be captured at all, and need no defense even if attacked.