The term 'mechanism' has been used in two quite different ways in the history of biology. Operative, or explanatory mechanism refers to the step-by-step description or explanation of how components in a system interact to yield a particular outcome (as in the 'mechanism of enzyme action' or the 'mechanism of synaptic transmission'). Philosophical Mechanism, on the other hand, refers to a broad view of organisms as material entities, functioning in ways similar to machines--that is, carrying out a variety of activities based on known chemical and physical processes. In the early twentieth century philosophical Mechanism became the foundation of a 'new biology' that sought to establish the life sciences on the same solid and rigorous foundation as the physical sciences, including a strong emphasis on experimentation. In the context of the times this campaign was particularly aimed at combating the reintroduction of more holistic, non-mechanical approaches into the life sciences (organicism, vitalism). In so doing, Mechanists failed to see some of the strong points of non-vitalistic holistic thinking. The two approaches are illustrated in the work of Jacques Loeb and Hans Spemann.