 Aristotle introduces his notions of matter and form in the first book of his Physics, his work on natural science. Natural science is concerned with things that change, and Aristotle divides changes into two main types: there are accidental changes, which involve concrete particulars, or “substances” (ousiai) in Aristotle’s terminology, gaining or losing a property (see Categories 1–5, Physics i 7). For instance, the changes whereby Socrates falls in a vat of dye and turns blue, or puts on a few pounds from excessive feasting during the Panathenaia, count as accidental changes (in the categories of quality and quantity, respectively). Socrates, a substance, gains the property of being blue, or the property of weighing twelve stone. The other main kind of change is substantial change, whereby a substance comes into, or passes out of, existence. For example, when Socrates dies, or is born (or perhaps conceived, or somewhere in between conception and birth), a substantial change has taken place.