 As well as purely textual arguments, several more philosophical motivations have been proffered in favour of matter-involving forms. One such argument relies on the fact that natural things, unlike mathematical ones, are subject to change. Only things with matter are capable of change, and, if natural forms are to account for the characteristic changes undergone by natural compounds, the claim is that they must themselves be matter-involving. For example, the property of falling downwards when unsupported is one had by all human beings. Aristotle would explain this propensity as being due to their being made of a preponderance of the heavier elements, earth and water. If the form of a human being is to account for this fact, plausibly it will have to make mention of the material constitution of human beings that results in this sort of characteristic behaviour.