In composing Metaphysics, Aristotle mainly aimed to explore the most fundamental ontological questions, that is the "causes and principles" ofbeing. The books Beta, Gamma, Epsilon, Zeta, Eta, and Theta clearly explore ontological topics, especially the issue of existence. The ideas in Metaphysics do have a certain conceptual unity. All the books contain discussions relating to metaphysics. Metaphysics also exerted a great influence on early Islamic philosophers. Aristotle's account of substance as a form present in matter rivaled the earlier Platonic idea that substances must be incorporeal, timeless forms existing outside the tangible world. In the modern period, however, a critique issued by the eighteenth-century Scottish philosopher David Hume challenged the work's relevance. Aristotle believed that particular properties of a man, such as a snub nose or height, do not explain what it is to be a man. But Hume argued the opposite. This criticism did not erase Aristotle from philosophical debate completely, but it did substantially diminish his influence.