The Law of Causality

The law of causality is a corollary of the law of identity, which states that every action has a cause (the cause being the nature of the entity which acts); and the same cause leads to the same effect (the same entity, under the same circumstances, will perform the same action).

The validation of this law rests on two points: that all action is action of entities and the law of identity. Every entity has a specific, non-contradictory, limited nature; it has certain particular attributes and no others. Such an entity must act in accordance with its nature: the only alternative to this is for an entity to either act apart from or against its nature, both are impossible.

Entities cannot act apart from their nature because existence is identity: anything that exists has identity, a thing apart from its own nature would not and could not exist. Entities cannot act against their own nature because A is A, therefore contradictions are impossible---they are what they are, not what they are not, for an entity to act against its nature means for it to be what it is not.

The implication of the above is that at any given time an entity has one and only one action available to it: namely the action caused by its nature. For an entity to have multiple incompatible actions would mean that it has multiple incompatible (contradictory) aspects of its nature which each cause one of the possible actions, and this would mean that the entity would lack identity and would therefore not exist.

In Ayn Rand's words:

The law of causality is the law of identity applied to action. All actions are caused by entities. The nature of an action is caused and determined by the nature of the entities that act; a thing cannot act in contradiction to its nature.^[1]