First, a word about the idea that the term level is used as a metaphor. According to the Oxford English dictionary, the term “level” derives from terms describing the idea of being parallel to the ground. From there, the term could easily extend to the idea that a building can have multiple levels, or landings, from the ground up. Something like this spatial arrangement of stacked landings seems to be the basis for many applications of the level metaphor (in other words, landings are the “secondary subject” of the metaphor). Usage of the term “level” increased dramatically in the 20th Century, and surely some of this increase is explained by the fecundity of the metaphor for ordering items in a set from the highest to the lowest (under some understandings of “high” and “low”). These are the specific topics (the primary subjects) to which the secondary subject (landings) is compared. In asserting that the term “level” has this metaphoric aspect, I do not intend to thereby disqualify the metaphor from also expressing a set of true or false commitments about the structure of the world. Indeed, I think that the mechanistic application of the metaphor more or less accurately describes the structure of at least many systems in the domains of the special sciences.
My point in being a pluralist about the levels metaphor is simply to emphasize that the metaphor of a horizontal landing is used to describe a wide variety of altogether distinct primary subjects. As a result, any understanding of how this level metaphor works, what it is committed to, and whether it works for a particular purpose, must begin with a clear understanding of how the metaphor is unpacked in the given application. (One cannot, to pick a perfectly clear example, equate the great chain of being and levels of mechanisms.) A second point was to emphasize that there are many legitimate applications of the metaphor, and that the utility of the metaphor might vary from one application to the next. I am not arguing for a single, correct way of understanding the level metaphor. However, I am arguing that one prominent use of the level metaphor in sciences such as neuroscience can be usefully unpacked as describing mechanistic levels.