 Philosophers David M. Armstrong and Nancy Cartwright have argued that the actual laws of nature are oaken rather than iron (to use Armstrong’s terms). (Armstrong 1983; Armstrong 1997, 230–231; Cartwright 1983). Oaken laws admit of exceptions: they have tacit ceteris paribus (other things being equal) or ceteris absentibus (other things being absent) conditions. As Cartwright points out, an inference based on such a law of nature is always defeasible, since we may discover that additional phenomenological factors must be added to the law in question in special cases. 