B, the are four possible relationships: A causes B (direct causation); B causes A (reverse causation); A and B are both caused by C (common causation); There is no connection between A and B; the correlation is a coincidence. These relationships are not mutually exclusive; they may exist in any combination. For example, it is possible that both A can cause effect B and B can cause effect A (bidirectional or cyclic causation). No conclusion can thus be made regarding the existence or the direction of a cause-and-effect relationship only from the fact that A and B are correlated. Determining whether there is an actual cause-and-effect relationship, and if so which direction the causality is, requires further investigation. If the relationship between A and B is statistically significant, the final relationship in the list above ("coincidence") may be statistically ruled out, but the correlation itself will not clarify whether A caused B, B caused A, or A and B were both caused by some other effect, C. In philosophy and physics The nature of causality is systematically investigated in several academic disciplines, including philosophy and physics. In academia, there are a significant number of theories on causality; The Oxford Handbook of Causation encompasses 770 pages. Among the more influential theories within philosophy are Aristotle's Four causes and Al-Ghazali's occasionalism. David Hume argued that beliefs about causality are based on experience, and experience similarly based on the assumption that the future models the past, which in turn can be based only on experience, which leads to circular logic. In conclusion, he asserted that causality is not based on actual reasoning: only correlation can actually be perceived. Immanuel Kant, according to , held that "a causal principle according to which every event has a cause, or follows according to a causal law, cannot be established through induction as a purely empirical claim, since it would then lack strict universality, or necessity". Outside the field of philosophy, theories of causation can be identified in classical mechanics, statistical mechanics, quantum mechanics, spacetime theories, biology, social sciences, and law. To establish a correlation as causal within physics, it is normally understood that the cause and the effect must connect through a local mechanism (cf. for instance the concept of impact) or a nonlocal mechanism (cf. the concept of field), in accordance with known laws of nature. From the point of view of thermodynamics, universal properties