Action, in physics, is a quantity with the dimensions of energy × time. It is most commonly encountered in modern physics in connection with Planck's constant, which has these dimensions and is therefore sometimes called Planck's quantum of action. Pairs of measurable quantities whose product has dimensions of energy × time are called conjugate quantities in quantum mechanics, and have a special relation to each other, expressed in Heisenberg's uncertainty principle. In classical physics, the term originated in the 17th century and assumed a variety of different meanings to do with the evolution of a system between two defined states. It crops up in the statement of Newton's third law "Action and reaction are equal and opposite," although the interpretation of this is unclear unless "action" is defined.