Religion is a category concept used to sort social practices, a taxon that is widely recognized in the humanities and the social sciences. Its wide usage, however, raises philosophical issues that are probably no different than those that afflict other abstract concepts that are used to sort cultural types, such as “literature” or “democracy”.

Many approaches to the study of religion use a concept of essence to distinguish what is and is not religious. These are called monothetic approaches because they suggest that there is one or a limited number of properties that define a religion. As such, they are a form of the classical view that every instance of a phenomenon will share a defining property and thus be accurately described by a concept with that identifying property.

Some scholars, notably Emile Durkheim, argue that religion is not a substantive but rather a functional concept that explains the unique role that certain sets of social practices play in human societies. When a society has enough rituals and practices that unite people into a moral community, they can be defined as having a religion. These functional definitions, sometimes termed “monothetic-set” definitions, have become the dominant way of thinking about religion in the twentieth century.

Other scholars, mainly in the humanities, have pulled the camera back and are looking at the way that these ideas about what makes something a religion are constructed. These are often referred to as reflexive approaches to religion, although they can be applied to other cultural phenomena as well.