In reviews of manuscripts and grants, the words “mechanistic” and “descriptive” are often misused as synonyms for “good” and “bad,” respectively (6, 9). The extraordinary power of these words requires us to wield them carefully when critiquing science. In an earlier essay, we considered the epithet “descriptive” as applied to science and argued for an important role of descriptive studies in biology, while also acknowledging a general preference for studies that go further by including experimental work (3). Here we consider the more favored adjective “mechanistic” and explore its usage, meanings, implications, and limitations. Recognizing the centrality of mechanistic research to the history of science (1), we seek to explore what biological scientists mean when they use this term. Definitions. At first glance, one is struck by the fact that the terms “descriptive” and “mechanistic” are often used antagonistically as descriptors of scientific quality, yet they are not antonyms.“Descriptive” is defined as “referring to, constituting, or grounded in matters of observation or experience,” while “mechanism” is defined as “the fundamental processes involved in or responsible for an action, reaction or other natural phenomenon”(http://www. merriam-webster. com/dictionary/). From these definitions,“descriptive” can be seen as analogous to the interrogatives “who,”“what,”“where,” and “when,” whereas “mechanistic” in turn asks “how” and “why.” Hence, these terms collectively encompass the spectrum of inquiry. But if “descriptive” and “mechanistic” are not antonyms, what accounts for the general preference for mechanistic over descriptive work?

As “descriptive” and “mechanistic” denote different qualities, at least in the minds of reviewers, we must probe further to ascertain what these terms mean in the scientific vernacular. We suggest that these words mean different things to different people. Since practically all laboratory-based biological science is based on recording evidence from experimentation, an argument can be made that all science is in some form descriptive. However, this is unsatisfactory because every scientist intuitively knows that there are qualitative differences in scientific studies. Hence, the first problem we encounter is in the precision of language, as we try to understand and convey meaning in words. The word “mechanistic” is used to refer to both complex natural phenomena and man-made mechanical devices. The machine as an analogy for the natural world owes much to the writings of Hobbes and Descartes (even though the latter could not bring himself to ascribe the human soul to a mechanical process). Like Hobbes, the modern scientist makes the implicit assumptions that phenomena have rational explanations and that events may be connected as cause and effect. Scientists seeking mechanisms to explain the workings