Religion is a system of rules for sanctioning and rewarding, appraising and disapproving, and inspiring and ideationalizing behaviour. It is also a community within which people share values, attitudes, and expectations about the world. It can be a social glue: research supports Emile Durkheim’s theory that it is the major force (some think the only one) that holds society together.

The value systems which religions promote are extremely varied. They may be cosmological, involving beliefs in disembodied spirits or heavenly orders of life; or they can be moral, promoting ethical behaviours and practices; or they can be natural, in which case they grow out of human questions about the universe and the way it works. Some of the latter are philosophy-like, like Buddhism developed by Siddartha Gautama (563–483 bce). Others, such as Christianity and Judaism, grow out of specific divine messages from God or a particular prophet.

People act religiously in many ways, scrupulously, generously, ecstatically, sacrificially, prayerfully, puritanically, and ritualistically. Psychologists, who study the human mind, argue that religion answers basic needs, such as a fear of death or a desire for meaning in life; and neuroscientists claim that there is a brain region which is devoted to religious experiences.

The vast variety of practices that are now said to fall under the concept of religion raises several issues. The first of these concerns is whether it is possible to understand a social genus in terms of necessary and sufficient properties, or if we should instead view it as a family-resemblance concept.