adam smith reminds us ( his assessments perhaps to some degree coming from an understanding of the period immediately after the black death when survivors of the plague , no matter how indigent , were able to demand higher salaries for themselves , which brought about the middle class for the first time in history,135 as from his thorough study of commerce ) that professions are awarded higher wages based upon the rarity of the knowledge and skills by which certain products and services are rendered.136 and likewise , life itself , if it were rare , would have greater innate value at least in human estimations , but its fecundity does not allow that to happen .