 A world is a complete specification of a causal model; the details will depend upon the type of model. For now, we note that a world will include, inter alia, an assignment of values to all of the variables in the model. If the variables represent the properties of individuals in a population, a world will include an assignment of values to every variable, for every individual in the population. A variable can then be understood as a function whose domain is a set of worlds, or a set of worlds and individuals.