The Mirror of Zen

There are ten sickness to avoid in hwa-du practice: trying to
figure out the kong-an using discriminative thought, seizing on
the master's wordless teaching gestures, such as raising his eyebrows
or winking, allowing yourself to get caught by words and
speech, searching for proofs or evidence in the kong-an, miming
the shout or sudden expression of some masters as if it were your
own thing, abandoning everything by falling into emptiness,
attempting to distinguish between conditions of existence or
nonexistence, thinking in terms of absolute nothingness, knowing
things in terms of logical reasoning, and impatiently expecting
awakening.
You must completely avoid these ten sicknesses! Instead,
firmly holding the Great Doubt inspired by the kong-an, and keeping
your mind clear, pour all of your energy into the question "What is this?"