A fair rule should remain recognizable when applied to someone you dislike.

The real test of a standard is how it looks applied to someone you would rather see lose. If it suddenly seems too rigid, or too generous, or in need of an exception, the standard was never doing the work; your preferences were. This is uncomfortable because almost everyone fails the test occasionally, and almost nobody notices themselves failing it.

When you are about to defend a rule or attack one, do a quick swap. Imagine the same rule applied to the person you are least sympathetic to, or to your closest ally. If your view changes, the position you hold is about people rather than principles, and it is worth knowing that before you argue for it.