Logical NOT is a unary operation that inverts a binary value: 0 becomes 1, and 1 becomes 0. It lets one machine reverse the decision produced by another. section 7.2.1
Unauthorized-person alarm
A face detector outputs 1 when it recognizes an authorized face and 0 otherwise. Applying logical NOT reverses that result, allowing an alarm system to respond to an unrecognized face. section 7.2.1

