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An ontology is a tool: a way of relating to the world that enables activities we care about.
Formal rationality depends on a perfectly sharp ontology, because that makes absolute truths possible. Truths, like “HIV causes AIDS,” often enable activities we care about, like disease prevention and treatment. In a formal ontology, things definitely belong to a category or don’t, properties have precise values, reality is tidily divided into objectively separable individuals, and they are in unambiguous relationships with each other. All this should hold independent of context and purposes; if something is an eggplant, it remains an eggplant wherever you take it and whatever you do with it. Rationalism assumes, implicitly or explicitly, that the world actually works this way.
At the eggplant scale, it doesn’t, due to the many manifestations of nebulosity we explored in Part One. The world as described by quantum physics is independent of context and purposes. However, that ontology is useful only in rare circumstances, for unusual purposes like nanoscale device design, so it is also contextual and purpose-laden from our point of view. There is no uniform, accurate, context-free, purpose-free, objective ontology for eggplants, baldness, or river crossing. The eggplant-sized world is indefinite in roughly the same way language is.
Reasonableness works with nebulous, tacit, interactive, accountable, purposeful ontologies and truths, which enable everyday routine activity. (And, we’ll see in Part Three, they’re also necessary to make rationality work at the eggplant scale.)
Nebulous means that something can be “pretty much” an eggplant, without there being any ultimate truth of the matter. Tacit means that use of an ontology generally goes unnoticed and unexpressed. Interactive means that ontology is an aspect of activity; you treat something as an eggplant (or not) in the course of shopping or cooking. Accountable means that if you treat something iffy as an eggplant, you may be expected to give an explanation of why it counts as one. Purposeful means that ontologies are tools for getting work done, you use different ones on different occasions, and whether or not something counts as an eggplant depends on what you are doing with it.