Nebulosity is the central concept for In the Cells of the Eggplant. The book’s fundamental theme is “how does rationality relate to nebulosity, and how can we make that work better?”
Literally, nebulosity means “cloud-like-ness”:
- Boundaries: Clouds do not have sharp edges; they thin out gradually at the margin. As you approach a cloud (in an airplane, or on a mountain hike), you cannot say quite when you have entered it.
- Identity: It may be impossible to say where one cloud ends and another begins; whether two bits of cloud are part of the same whole or not; or to count the number of clouds in a section of the sky.
- Categories: Cirrocumulus shades into cirrus and into altocumulus; clouds of intermediate form cannot meaningfully be assigned to one or another.
- Properties: Depending on temperature and density, clouds may be white, gray, blue, or iridescent. There are no specific dividing lines between these colors. Clouds have diverse, highly structured shapes, which cannot be precisely described. First, because the edges are indistinct; and second because the shape is so complex that a full description would be overwhelmingly gigantic even were it possible. Yet meteorologists find useful phrases like “ragged sheets,” “wavy filaments,” “bubbling protuberances,” and “castle-like turrets.”
Clouds are an extreme case, but nebulosity is pervasive. Other than in mathematics and fundamental physics, nothing is ever definitely this-or-that. Everything is always somewhat this and somewhat that. Put under high enough magnification, a stainless steel ball exhibits the same indefiniteness as a cloud. No ball can be perfectly round, nor made of perfectly pure steel, nor can one definitely say whether some particular atoms are part of it or part of its surrounds.
“Gray areas” are the easiest way to think about nebulosity. Is it true that maroon is a shade of red? Well, pretty much, although it’s a bit of a gray area.
Questions of degree do not exhaust nebulosity, though. Consider “yes, there is some water in the fridge: in the cells of the eggplant.” The problem here is not one of a gray area between water and non-water. Nor are we factually uncertain whether or not there is water in the fridge, so scientific investigation could answer the question. It is not that the word “water” has two different meanings, referring to two different kinds of water. It is that what counts as water depends on what you want it for.
Part One of The Eggplant explains in detail why there are no “really truly true truths” about eggplant-sized objects. Mostly, the best we can get is “true for all practical purposes.” And most of the truths we use, even in the hard sciences, are “pretty much true” or “true as far as a particular purpose goes.” This raises occasional problems for rationality in practice, and causes serious difficulties for rationalism as a theory.