The Protestant Reformation presented another severe challenge to orthodoxy. In this context, the assault on Aristotle's natural philosophy began in earnest. Some early answers equated everything with some element of the world, such as water. Aristotle classified the possible answers to this question as emphasizing either the form or the matter of things. Aristotle understood the ancient atomists as claiming that the nature of things we observe is found in their constituents, the atoms.
Atoms are tiny, hard particles which move about in otherwise empty or void space.
Insofar as the atoms are the matter making up things in the world, the atomists are materialists. Aristotle rejected atomism entirely. He advanced arguments against the void, but more generally he held that the nature of things is their form. At the other extreme were philosophers who emphasized form to the extent that matter becomes almost an afterthought. The Pythagorean philosophers found numerical forms in all things.