One crucial asymmetry of causation concerns action. Knowing the causes of an event helps one to bring it about. Knowing its effects does not. Knowing what things are related to an event as effects of a common cause does not help one to bring it about either. Causes are means and tools. People can use them to bring about their effects. This observation suggests a theory of causal asymmetry – something like: a causes b if and only if a can be used as a means to bring about b. This view has appealed to scientists and to statisticians. For example, the economist Guy Orcutt writes, “… we see that the statement that … z1 is a cause of z2, is just a convenient way of saying that if you pick an action which controls z1, you will also have an action which controls z2” (1952, p. 307).

Such a theory links the asymmetry of causation to a method of determining which way the causal arrow points: If there is a correlation between a and b and one wants to know what the causal connection is, intervene to find out. a's cause b's just in case human interventions that bring about tokens of kind a lead tokens of kind b to occur as well, while human interventions that bring about b's do not lead to tokens of a occurring. This chapter will be concerned with this theory of causal asymmetry. Following Price and Menzies, I shall call it “the agency theory.” It might also be called “the manipulability theory.”