Complex sentence: A person's life stance, or lifestance, is their relation with what they accept as being of ultimate importance, the presuppositions and theory of this, the beliefs, commitments and practice of working it out in living.
Split into simpler sentences: A person's life stance, or lifestance, is their relation with what they accept as being of ultimate importance. It involves the presuppositions and theory of, the beliefs, commitments and practice of working out in living.