Finite spirits can be plausibly viewed as entities postulated by a theory, comparable to the position on mental states and processes developed in the latter part of the twentieth century. This position is developed here by reference to the account in the synoptic gospels of the exorcism of the Gadarene demoniacs. The role played by specifying causal relationships between postulated entities and objects whose existence is not in doubt is examined. Also, various features of theories are discussed in relation to this example, viz. theory-laden description, classifying theories as naturalistic or supernaturalistic, kinds of evidence, and the importance of the method of hypothesis (or abduction) in critically scrutinizing the claims of religion.