Causality is the principle that a precisely determined set of initial conditions will always result in the same effect at a later time. Causality was a basic tenet of classical physics which assumed, both physically and philosophically, that it would always be possible to establish the initial state of a system to any desired degree of accuracy. The impossibility of such a precise determination and the consequent breakdown of causality at the subatomic level is a fundamental result of quantum mechanics and, in particular, of the Heisenberg uncertainty principle.


In relativity theory one event can have a causal influence on another only if energy or information sent from the first reaches the second. See also causal structure.

Causality and unity
"Science is all about cause. Now, if really everything is connected to everything, if there really is only a oneness, everything then affects everything, and the whole idea of causality has to be revised."

– Willis Harman, Metaphysics and Modern Science


Causality on a large scale would be undermined should some means be found to (1) open a portal enabling matter to travel backward in time, or (2) send a message into the past, or, equivalently, at a speed faster than that of light. Just such a breakthrough was claimed in a paper in Physical Review Letters (May 22, 2000) based on the work of Anedio Ranfagni of the Italian National Research Council. See also time and time travel.


Causality in philosophy

According to Aristotle everything has four causes: the material cause, the material substance involved; the formal cause, its shape or structure; the efficient cause, the agency which imposes the shape upon the matter, and the final cause, the end to which it is done.


The Scottish philosopher David Hume held that causes and effects are not real, but instead are imagined by our minds to make sense of the observation that A often occurs together with or slightly before B. All we can actually observe are correlations, not causations. This is also expressed in the logical fallacy, "correlation implies causation." Kant, on the other hand, held that causality was one of the a priori categories necessary for the ordering of experience.



1. Krasnikov, S. V. "Causality Violations and Paradoxes." Physical Review D, 55 (6): 3427-3430 (1997).