Actioni contrarium semper et aequalem esse reactionem: sive corporum duorum actiones in se mutuosemper esse aequales et inpartes contrarias dirigi. To everyaction there is always opposed an equal reaction: or, themutual actions oftwo bodiesuponeach other are always equal, and directed to contrary parts.

-Newton, Sir Isaac
   Third Law of Motion. Philosophiae Naturalis Principia Mathematica (translated by Andrew Motte,1729).

   Vengeance is a personal reaction. But not one that government can indulge in.

-Reno,Janet
  Opposing the death penalty. In the NewYorkTimes,15 May.

Television has spread the habit of instant reaction and has stimulated the hope of instant results.

-Schlesinger, Arthur M(eier),Jr
  In Newsweek, 6 Jul.