 The interactions of particles too small to observe is a compelling way to account for perceptible changes in the natural world. Even Aristotle—often cast as the arch-enemy of atomism—allowed that there might be a lower limit to the quantity of matter that could instantiate certain properties. But not all atomist theories were based on an appearance/reality distinction: Buddhist philosophers posited phenomenal instants with minimum extension in time as well as space, to mirror the ephemerality of moments of human experience. Void spaces between atoms sometimes, but not always, feature in atomist theories.