And despite the advent of alternative forces, Durkheim argued that no replacement for the force of religion had yet been created. He expressed his doubt about modernity, seeing the modern times as "a period of transition and moral mediocrity." Durkheim also argued that our primary categories for understanding the world have their origins in religion. It is religion, Durkheim writes, that gave rise to most if not all other social constructs, including the larger society. Durkheim argued that categories are produced by the society, and thus are collective creations. Thus as people create societies, they also create categories, but at the same time, they do so unconsciously, and the categories are prior to any individual's experience. In this way Durkheim attempted to bridge the divide between seeing categories as constructed out of human experience and as logically prior to that experience. Our understanding of the world is shaped by social facts; for example the notion of time is defined by being measured through a calendar, which in turn was created to allow us to keep track of our social gatherings and rituals; those in turn on their most basic level originated from religion. In the end, even the most logical and rational pursuit of science can trace its origins to religion. Durkheim states that, "Religion gave birth to all that is essential in the society." In his work, Durkheim focused on totemism, the religion of the Aboriginal Australians and Native Americans. Durkheim saw this religion as the most ancient religion, and focused on it as he believed its simplicity would ease the discussion of the essential elements of religion. As such, he wrote:Now the totem is the flag of the clan. It is therefore natural that the impressions aroused by the clan in individual minds—impressions of dependence and of increased vitality—should fix themselves to the idea of the totem rather than that of the clan: for the clan is too complex a reality to be represented clearly in all its complex unity by such rudimentary intelligences.Durkheim's work on religion was criticized on both empirical and theoretical grounds by specialists in the field. The most important critique came from Durkheim's contemporary, Arnold van Gennep, an expert on religion and ritual, and also on Australian belief systems. Van Gennep argued that Durkheim's views of primitive peoples and simple societies were "entirely erroneous". Van Gennep further argued that Durkheim demonstrated a lack of