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Concept in the game of Go
the concept in the game of Go

Life and death (死活) is a fundamental concept in the game of Go, where the status of a specific group of stones is determined as either being "alive", where they may remain on the board indefinitely, or "dead", where the group will be "captured" and removed from the board. The basic idea can be summarized by:
A group must have a means to evade capture forever by having at least two liberties that can't be filled at the same time (by having at least two "eyes") to sustain itself.

The concept of Life and death is a consequence of two basic Go rules. A group of stones with no liberties is captured. A stone may not be played on a position where it will be captured directly, unless it directly captures another group. Because of these rules, some groups can never be captured (and thus are alive), while others cannot avoid capture (and thus are dead even if they have not yet been removed from the board). The group at a is not alive, but may become alive, so is unsettled. The group at b is dead. The group at c and d is alive.

Liberties inside a group are called eyes. In the previous example, a, b, c and d are eyes. An eye may be a single empty intersection, or more of them. In the diagram on the left, the intersections marked e form a single eye, and f forms another eye. The group has two—separated—eye spaces. Even though white can make a move in these eyes, black could eventually capture the white stones, and each eye would thereby be reduced to a single intersection. An inner liberty where the opponent can still capture part of the group that encloses this liberty is called a 'false eye' or 'deficient eye'. A group with only one eye is not alive. In the example on the left, white can play at g, and capture a black stone. That eye is then destroyed, and black has only one eye left, so that group is now dead. The black group at g is dead, because it can not form two true eyes.

Life and death situations and issues occur when an area with a group of stones surrounds a small area (<7 points) so that it may not be possible to form two separate independent "eyes". As the board fills up during the course of the game, certain groups will survive, and others may not. A group with a single eye can normally be captured, in the end, by filling first round the outside. The purpose of making two eyes is to prevent this. Novices sometimes interpret making two eyes in a narrow way, and form 'explicit' eyes one by one. This is often the wrong approach, and it is better to play generally to make a territory inside a group out of which two eyes can surely be made, if and when the opponent attacks it. Groups with seven or more points of territory can usually form two eyes when attacked, unless there are structural weaknesses. Because the loss of a group can mean the loss of the game, and because the efficient use of each move is important, knowing the life and death status of one's own groups (as well as one's opponent's) is an important skill to cultivate, if one is to become a strong player. The correct, accurate plays with which to make a group secure, or to kill the opponent's group, are studied deeply by all strong players.

The concept of 'status' is discussed in Life and Death by James Davies. Groups of stones are divided into those that are alive, dead or unsettled. Here alive, for example, is an unconditional judgement made, that with best play from both players, the group can survive to the end of the game. That assumes the opponent starts: the alive status means that, whatever the attack made, there is an adequate defensive answer. The unsettled status is therefore most worthy of attention. By playing first, the attacking player can kill such a group. By playing first, the defending player can save such a group. There is an enormous range of formations that are unsettled. A group can be considered "alive", "dead", or "unsettled" based on whether two eyes can be made regardless of how the opponent plays. Naturally, one space is insufficient. Two adjacent spaces are insufficient to make two eyes as well, since putting a stone in will create one eye only. The opponent can simply fill in one of the two spots, threatening to fill in the other, forcing the player to capture that one by playing in the second one. Then, the opponent simply plays there again, killing the group. Usually this will not be done during the game, but at the end, during calculation, the group will be labeled "dead" and removed as prisoners. The only way to actually remove a group from the board is to fill its "eye space" with all but one, forcing it to capture the filling in group by putting a stone in the last space. If the created group is not alive, then continuing this process will eventually kill the entire group.

Groups that are alive may yet die. One reason is that they may be sacrificed, in the course of a ko fight. Another way in which the ko rule enters the discussion is through the complications ko adds to the classification by status. It is quite possible for a group to be alive in ko: that is, the group is conditionally alive, the condition being to be able to win a particular ko fight relating to the control of a key intersection.

Virtually all games will have at least a few dead stones, which remain on the board at the end of the game, when both players pass. Those dead stones are then removed, in an operation often called 'cleaning', which is a separate phase of the game. The stones removed are treated exactly like other captured stones. Under Chinese rules, which use area counting, stones removed during the cleaning phase are returned to their bowls. It is a novice mistake to carry out the capture of dead stones before it is of tactical importance to do so. Such plays, during the game, waste a turn and may also cost points. Single stones and small groups are often sacrificed. In cases where a group is more than of sacrificial value, that group typically must make life in order for one to have a chance at winning the overall game. Generally each side will have at most 4 5 living groups on the board at the end of the game. There is a go proverb that says that "Five groups may live, but the sixth will die" which in a nutshell describes the need to emphasise connection between developing groups. The struggle for life can be solved by connection. Since each group needs two eyes, (and eyes are sometimes hard to come by) the alternative is to connect out to another group, thereby sharing both liberties and eyes.

Even if a group is lost, one can still use one's own dead stones for aji (potential). Ko threats are just one way in which apparently dead stones are put to good use. Expert players use a variety of 'squeezing' tactics, of which semedori, an advanced endgame technique, and shibori are two that have recognised Japanese language names.