 As Mackie (2000) points out, the full statement of this theory involves various relatively ad hoc restrictions. More importantly, it does not seem to explain what unifies the category of substance, for it does not say that it is true of any substance that it could conceivably be the only substance existing for a period of time—indeed, it could not apply to non-atomic substances, for complex ones can exist only if other substances—those that constitute their parts—also exist. It implies only that something from the category of substance might so exist and that this could not be true of any other category. One might wonder what enabled substances that could not exist alone to count as substances at all.