The failure rate of software: does it exist?

This paper first overviews some foundation issues in reliability and argues that generally, it does not make mathematical sense to talk about the failure rate of a piece of software. Furthermore, even when it does make sense, the failure rate is personal, and does not exist outside of the software engineer´s mind. Also discussed are matters pertaining to what constitutes a software reliability model, the Markov assumption underlying many of the existing models, the need for models with point masses, and the notion that under subjectivist thinking, reliability models are indexed by two time-scales: (1) mission time; and (2) time at which the software reliability is assessed.