A teleological explanation explains the presence or nature of some activity or entity by appeal to the goal or purposive that it subserves. The Etiological Theory of Function starts with two suppositions. The first is that all uses of teleological language in biology can be cast as instances of the form: The function of x in z is to y, where x is a type of process or structure and z is an organism type, and y is an activity. The second is that such locutions are by their nature explanations. The various accounts of function generally assume that there is a single set of uses to which we typically put function ascriptions in biology, and a single set of conditions under which their ascription is appropriate. Function ascriptions are univocal, but their explanatory uses are multifarious.