Not

!e returns True if e is False and False if e is True.

Attributes[Not] := {Protected}

Simple examples

In[1]:= Not[True]
Out[1]= False

In[2]:= Not[False]
Out[2]= True

In[3]:= Not[a]
Out[3]= Not[a]

In[4]:= Not[Not[a]]
Out[4]= a