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 systole and diastole of the heart; in the undulations of fluids and of sound; in the centrifugal and centripetal gravity; in electricity, galvanism, and chemical affinity. Super-induce magnetism at one end of a needle, the opposite magnetism takes place at the other end. If the south attracts, the north repels. To empty here, you must condense there. An inevitable dualism bisects nature, so that each thing is a half, and suggests another thing to make it whole; as, spirit, matter; man, woman; subjective, objective; in, out; upper, under; motion, rest; yea, nay.

"Ralph Waldo Emerson"

A page on the law of Polarity.