4.2: Pseudo-Variables

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In Pharo, there are 6 reserved keywords, or pseudo-variables: nil, true, false, self, super, and thisContext. They are called pseudo-variables because they are predefined and cannot be assigned to. true, false, and nil are constants, while the values of self, super, and thisContext vary dynamically as code is executed.

true and false are the unique instances of the Boolean classes True and False. See Chapter : Basic Classes for more details.
self always refers to the receiver of the currently executing method.
super also refers to the receiver of the current method, but when you send a message to super, the method-lookup changes so that it starts from the superclass of the class containing the method that uses super. For further details see Chapter : The Pharo Object Model.
nil is the undefined object. It is the unique instance of the class UndefinedObject. Instance variables, class variables and local variables are initialized to nil.
thisContext is a pseudo-variable that represents the top frame of the execution stack. thisContext is normally not of interest to most programmers, but it is essential for implementing development tools like the debugger, and it is also used to implement exception handling and continuations.