From The Jargon File (version 4.4.7, 29 Dec 2003) :
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.
From The Free On-line Dictionary of Computing (30 December 2018) :
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]