Sometime before 330 BC Aristotle asserted that the elements of fire, air, earth, and water were not made of atoms, but were continuous. Aristotle considered the existence of a void, which was required by atomic theories, to violate physical principles. Change took place not by the rearrangement of atoms to make new structures, but by transformation of matter from what it was in potential to a new actuality. A piece of wet clay, when acted upon by a potter, takes on its potential to be an actual drinking mug. Aristotle has often been criticized for rejecting atomism, but in ancient Greece the atomic theories of Democritus remained "pure speculations, incapable of being put to any experimental test".
Aristotle theorized minima naturalia as the smallest parts into which a homogeneous natural substance (e.g., flesh, bone, or wood) could be divided and still retain its essential character. Unlike the atomism of Democritus, these Aristotelian "natural minima" were not conceptualized as physically indivisible.
Instead, Aristotles concept was rooted in his hylomorphic worldview, which held that every physical thing is a compound of matter (Greek hyle) and of an immaterial substantial form (Greek morphe') that imparts its essential nature and structure. To use an analogy we could pose a rubber ball: we could imagine the rubber to be the matter that gives the ball the ability to take on another form, and the spherical shape to be the form that gives it its identity of "ball". Using this analogy, though, we should keep in mind that in fact rubber itself would already be considered a composite of form and matter, as it has identity and determinacy to a certain extent, pure or primary matter is completely unformed, unintelligible and with infinite potential to undergo change.
Aristotle's intuition was that there is some smallest size beyond which matter could no longer be structured as flesh, or bone, or wood, or some other such organic substance that for Aristotle (living before the invention of the microscope) could be considered homogeneous. For instance, if flesh were divided beyond its natural minimum, what would be left might be a large amount of the element water, and smaller amounts of the other elements. But whatever water or other elements were left, they would no longer have the "nature" of flesh: in hylomorphic terms, they would no longer be matter structured by the form of flesh; instead the remaining water, e.g., would be matter structured by the form of water, not by the form of flesh.