 The Manual of Reason (TS 10–14) repeats the conventional Vaiśeṣika theory that the first four substances (earth, water, air, fire) are each of two types, atomic and composite. An atom (paramāṇu) is indestructible (anitya), indivisible (i.e. non-composite), and has a special kind of dimension, called “small” (aṇu). The Vaiśeṣikas’ standard argument for atomism is as follows. It is an empirically established truth that whatever is perceived is composite. Thus even the smallest perceptible thing, namely, a fleck of dust in a sun-beam, has parts, which are therefore invisible. The Vaiśeṣikas call the smallest perceptible thing a “triad” (tryaṇuka) and claim that it has three parts, each of which is called a “dyad” (dyaṇuka). Does each of these parts itself have parts? Yes—for it is another empirically established truth that the parts of a visible thing themselves have parts (e.g. a piece of cloth, whose parts, the threads, are themselves composite). The Vaiśeṣikas say that a dyad has two parts, each of which is an atom.