While it is often remarked that "history repeats itself", in cycles of less than cosmological duration this cannot be strictly true. In this interpretation of recurrence, as opposed perhaps to the Nietzschean interpretation, there is no metaphysics. Recurrences take place due to ascertainable circumstances and chains of causality. An example is the ubiquitous phenomenon of multiple independent discovery in science and technology, described by Robert K. Merton and Harriet Zuckerman. Indeed, recurrences, in the form of reproducible findings obtained through experiment or observation, are essential to the natural and social sciences; and, in the form of chance observations rigorously studied via the comparative method, are essential to the humanities.