 Starting in the 1990s, a number of philosophers focused squarely on how the concept of a mechanism functions in science generally and molecular biology specifically (Glennan and Illari 2017; see also the entry on  mechanisms in science).  A number of characterizations of what a mechanism is have emerged over the years (Bechtel and Abrahamsen 2005; Glennan 2002; Machamer, Darden, and Craver 2000). Phyllis McKay Illari and Jon Williamson have more recently offered a characterization that draws on the essential features of all the earlier contributions: