Unlike space, time as we apprehend it has a direction. There is an asymmetry between the past (fixed) and the future (yet to exist). Time's arrow is whatever gives time this direction. Five aspects of the direction are: (i) that according to the second law of thermodynamics, disorder (entropy) increases from past to future; (ii) the universe is expanding in time; (iii) causal efficacy works in only one direction in time, since future events cannot influence past events (see backward causation); (iv) we remember past events whilst we cannot remember later ones; and (v) we can alter the future in a sense in which we cannot alter the past. A complete understanding of time would enable us to relate these five aspects of time's arrow, and to know whether they are true of necessity or only as a consequence of other contingencies. See a-series, time.