 Aristotle before Avicenna had also dedicated a long section of his Physics (4.6–9) to a refutation of the void. Aristotle’s arguments might all best be described as physical ones inasmuch as they all appeal to physical concerns associated with the void. More specifically, Aristotle’s arguments attempt to show that motion would be impossible on the assumption of a void. Avicenna reproduces many of Aristotle’s physical arguments, often giving them his own unique twist. Additionally, however, Avicenna develops what might be called a conceptual critique of the notion of void. In this critique, Avicenna attempts to show that the very idea of a void is conceptually vacuous or a vain intelligible. The criticism begins with a logical/physical principle: Anything that actually exists within the natural order can be defined in terms of a genus and difference. (The general idea is that the things that make up the physical world must be of some kind or species if they exist at all, and it is the genus and difference that identify the kind or species.) He then adds one further qualification: Inasmuch as these actual existents positively make up the world around us, their genus and difference must refer to positive (as opposed to negative) notions; a species properly understood answers the question “what is it?” and a genus or difference given in negative terms simply would not tell us what something is, but what it is not. (For a discussion of Avicenna’s theory of genus and difference and how it relates to his criticism of the void see McGinnis 2007b.)