In its barest of forms, the levels metaphor demands little of its object; it requires only a set of items and some criterion for ranking them as higher or lower than one another in some respect. Seniors are at a higher level in the American high school system than juniors, poetry is at a higher level than pushpin, lust is at a higher (and lower) level than like, and cells are at a higher level than molecules. In these examples, it is obvious that different kinds of thing are related by entirely distinct kinds of relation. In subtler cases, the equivocation is less noticeable, and for that, all the more misleading.
Three defining questions can be used to explicate how the levels metaphor is applied in a given context:
The Relata Question: What kinds of item are being sorted into levels?
The Relations Question: In virtue of what are two items at different levels?
The Placement Question: In virtue of what are two items at the same level?
The Relata Question provides an important clue about the intended sense of levels. The flexibility of the metaphor allows it to be applied to abstracta, such as branches of mathematics and ethical principles, or to concreta, such as astronomical objects and stereo equipment. The metaphor can be applied to types, such as sergeants and corporals, or to tokens, such as the relationship between Colonel Blake and Corporal O’Reilly. It can be applied to objects such as cats and mountains, to activities such as releasing neurotransmitters and making decisions, and to properties such as excitability or charge. Within the neurosciences, the levels metaphor is applied fluidly to causes, descriptions, developmental stages, events, explanations, scientific fields, objects, properties, techniques, and theories. Confusion arises when we assume that each application is the same.
The Relations Question concerns the ordering relationship by which items are said to be at a higher or a lower level than one another. A theory, for example, might be said to be at a higher level than a second if the first is derivable from the second (and not vice versa); the lowest-level theories are in this sense “fundamental.” Poetry might be said to be higher than pushpin in the sense that it requires greater intellectual skill and training to take pleasure in the former than to take pleasure in the latter. A technique might be said to be at a lower level than another because it detects phenomena at a smaller size scale. Some applications of the levels metaphor are discrete in the sense that there is a gap between things at lower and high levels; other applications are continuous, as when one uses the metaphor to describe size. We are unlikely to confuse such wildly different kinds of relationship. However, as we will see, the metaphor is used in other contexts where it is beguilingly difficult to keep them distinct, even for those who know better.
The Placement Question asks for the principle by which different items are located on the same level. Many uses of the levels metaphor rely at heart on an answer to the placement question. When the metaphor is used to describe size scales, for example, puffins and porcupines are at roughly the same level, vasopressin and oxytocin are at roughly the same level, and hydrogen and oxygen atoms are together at a lower level still. Juniors are all juniors because they are in their third year of American high school. For Marr, computational level questions are directed at what is computed and why it is computed that way (Shagrir 2010; Bechtel & Shagrir 2013).[2] Not every account of levels requires an answer to the placement question affirmatively. Indeed, it is of central importance that the idea of levels of mechanisms articulated here entails no positive story about what it means to be at a level, only a negative story about when things are not at different levels.