Function and forethought in design

The function or intended use of a device such as a fire escape is escape from fire, whether or not it is ever actually used as such or it acquires other uses. Biological function, on the other hand, depends, in one way or another, on actual use. Philosophical attempts to explain artifact function by analogy with biological function or to build a unified theory of function are nonetheless common. What such attempts routinely overlook, however, is the role of forethought in intentional function. I defend the traditional intended use view of function, emphasize the role of forethought and argue that design intentions are rationally grounded in a design brief, for which there is no obvious natural analogue.