Theories in certain scientific fields may be in flux today, but this unsettled state must be attributed to scientific progress, not to a lack of theoretical rigor. Several decades of scientific research have recently culminated in a wealth of new discoveries in these fields, and whenever many new facts come to light in a field, the ways in which that field organizes its knowledge inevitably need adjustment. The argument proceeds by ['showing that two alternate explanations for a situation are equally probable', 'citing a law of nature to explain a particular kind of change', 'explaining why a situation came about by referring to the intended outcome of a course of action', 'presenting the situation to be explained as part of a general pattern']