Alive
A group is alive when it can't be captured. [1]
There are several ways in which a group can 'live'. The most common are:
- the group has two eyes
- the group lives in seki.
- the group is secure enough to survive an invasion. [2]
[1] That is, the group as a whole isn't one that the opponent, with the move, can forcibly capture. Stones within it may be captured.
[2] A group that does not obviously have two eyes, or is not clearly in seki, is still alive if it is guaranteed to be able to get two eyes or make seki, regardless of who plays first. By this extended definition, groups that seem to be alive, or are alive can die if a player is negligent. See pass-alive, also ambiguously called unconditional life.
Being alive is one possible status of a group, as is being dead; in between groups are called unsettled, or in more open positions weak groups.
Based upon [2], I strongly opine, if not declare, that a group is alive if, and only if, its death could only come at the hands that group's color. In the diagram showing one simple example, would illustrate the sole mechanism by which the two-eyed black group could meet its death.
would otherwise just be an illegal suicide and therefore would not be placed before
. This would also be my answer to the argument TwoEyesCanDie. --ProtoDeuteric
Tactics for making live groups can combine threats: to make an eye, or to connect.
In this example the right-hand white group is alive, with two completely explicit eyes. The following diagram shows that the left-hand white group is also alive. The right hand group is of some help for that.
White can answer with
for two eyes.
here makes a false eye shape, but once White plays
all White's stones form one safe, live group.
See also the Eyes Collection.