Judge ye not

from the wise Dr. Ben Myers here (talking about a method of learning from the thoughts of great men of the past ):

.. If we learn from the past by distinguishing the timeless “perennial core” from the nonessential (i.e. flawed) elements, then we’re acting as though our own commitments are the final arbiter of history — we’re assuming that history has found its goal in us. And one of the unfortunate side-effects of this approach is that we’re no longer in a position to be critiqued by history. ...

The thing I can't understand is the side effect thing. May be what we choose to understand as the perennial elements are not correct and thus open us to critique ?