Eyes, life and death · Lesson 19
Why two eyes keep a group alive
An internal empty point surrounded by one side’s stones, which an opponent cannot freely enter, can form an eye. The black stones in this diagram are one connected group surrounding two separate eyes.
Even if every outside liberty is filled, an opponent entering one eye cannot capture Black because the other eye remains a liberty. The new opposing stone instead has no liberties, so the move is suicide. The opponent cannot play two stones at once; this black group lives.
Both eyes must be reliable and protected by the same connected group. Two adjacent empty points are not two independent eyes. Separate stones surrounding empty points may still be captured one group at a time.
Unconditional life means the opponent cannot kill the group even when playing first. Two true eyes are an easy example. Later we also meet seki, where both groups survive without each having two eyes.
Two real eyes
Original lessons by Yiban ·