In his discussion of the First Cause (or God), al-Farabi relies heavily on negative theology. He says that it cannot be known by intellectual means, such as dialectical division or definition, because the terms used in these processes to define a thing constitute its substance. Therefore if one was to define the First Cause, each of the terms used would actually constitute a part of its substance and therefore behave as a cause for its existence, which is impossible as the First Cause is uncaused; it exists without being caused. Equally, he says it cannot be known according to genus and differentia, as its substance and existence are different from all others, and therefore it has no category to which it belongs. If this were the case, then it would not be the First Cause, because something would be prior in existence to it, which is also impossible. This would suggest that the more philosophically simple a thing is, the more perfect it is. And based on this observation, Reisman says it is possible to see the entire hierarchy of al-Farabi's cosmology according to classification into genus and species. Each succeeding level in this structure has as its principal qualities multiplicity and deficiency, and it is this ever-increasing complexity that typifies the material world.