Source : Free On-Line Dictionary of Computing

the X that can be Y is not the true X
     
        Yet another instance of hackerdom's peculiar attraction to
        mystical references - a common humorous way of making
        exclusive statements about a class of things.  The template is
        from the "Tao te Ching": "The Tao which can be spoken of is
        not the true Tao."  The implication is often that the X is a
        mystery accessible only to the enlightened.  See the
        {trampoline} entry for an example, and compare {has the X
        nature}.
     
        [{Jargon File}]