Full context:   Aristotle considers the formal "cause" (εἶδος, eîdos) as the internal cause by which a thing is what it is. Often, form is confused with what Aristotelians call accidental form, i.e. what makes an inhering part what it is. Some say that form is the arrangement or shape of a thing, but this is not the case, since Aristotle would define these things as accidents, (falling under quality and position, respectively), while he defines form as the principle of a substance. This mistake is made because, with some things, what makes them what they are is their accidents. These things are called accidental beings. For things such as a three-legged stool, what makes it what it is is the number of legs it has, which is a quantity. But for other things, such as a bowl, if defined as a tool which has the ability to hold liquids, what makes it what it is is not its shape, but its ability to hold liquids, since its shape could change, and it could still have its ability.