An Etymological Dictionary of the German Language/Annotated/Vernunft
Vernunft, feminine, ‘reason, understanding,’ from Middle High German vernunft, Old High German firnunft, feminine, ‘activity of perception, sensual perception, comprehension, insight, understanding’; abstract of vernehmen, ‘to hear, perceive, understand,’ Middle High German vernëmen, Old High German firnëman, ‘to perceive, hear, experience, grasp, seize, understand.’ These figurative meanings are based on some such meaning as in Gothic franiman, ‘to take possession of, seize.’ For a similar evolution see vergessen (with different senses attached to the prefix); begreifen has been similarly developed in its figurative senses.