Polarity, or action and reaction, we meet in every part of nature; in darkness and light, in heat and cold, in the ebb and flow of waters, in male and female, in the inspiration and expiration of plants and animals; in the equation of quantity and quality in the fluids of the animal body; in the systole and diasystole of the human heart; in the centrifugal and centripetal gravity; in electricity, galvanism and chemical affinity.