Mastering Windows PowerShell Scripting - Third Edition

Not

The -not (or !) operator may be used to negate the expression that follows it.

For example, each of the following returns true:

-not $false 
-not (Test-Path X:\) 
-not ($true -and $false) 
!($true -and $false)

Double negatives The -not operator has an important place, but it is worth rethinking an expression if it injects a double negative. For example, the following expression will return true: -not (1 -ne 1). The preceding expression is better written using the -eq operator: 1 -eq 1.