 Monothetic approaches use a single property (or a single set of properties) as the criterion that determines whether a concept applies. The key to a monothetic approach is that it proposes necessary and sufficient conditions for membership in the given class. That is, a monothetic approach claims that there is some characteristic, or set of them, found in every religion and that if a form of life has it, then that form of life is a religion. Most definitions of the concept religion have been of this type. For example, as we saw above, Edward Tylor proposes belief in spiritual beings as his minimal definition of religion, and this is a substantive criterion that distinguishes religion from non-religion in terms of belief in this particular kind of entity. Similarly, Paul Tillich proposes ultimate concern as a functional criterion that distinguishes religion from non-religion in terms of what serves this particular role in one’s life. These are single criterion monothetic definitions.