Religion is notoriously difficult to define. Our modern use of the term derives from the 18th-century European Enlightenment, which regarded “religion” as a universal concept. During Europe’s colonial expansion — beginning in the 16th century and continuing into the early 20th — scholars encountered many unfamiliar worldviews that they chose to call “religions.”

The process was especially problematic with respect to Hinduism, which is sometimes called the oldest religion on earth but which other observers claim isn’t a single religion at all. Rather, they say, it’s a group of religions and philosophical movements (e.g., Shaivism, Vaishnavism and Shaktism) that are distinct, as a class, from those originating in other places such as the Middle East, China and sub-Saharan Africa. Unlike Christianity, Islam, Zoroastrianism and perhaps Judaism, “Hinduism” — compare Japanese Shinto — has no known “founder” and no rigid, easily definable core of common doctrines.

Some “religions,” such as Confucianism, might better be described as philosophies of life. Indeed, certain forms of Buddhism are nontheistic or even expressly atheistic. Thus, it’s quite incorrect to equate “religion,” as the word is commonly used, with “belief in God.”

And it’s manifestly wrong to say, as the late “New Atheist” writer Christopher Hitchens did, that “all religions are versions of the same untruth.” They’re far too varied for that. (Ironically, the vocal irreligiousness of the New Atheists turns out to be, on the whole, a peculiarly Abrahamic or Christian brand of unbelief. Indeed, one of them, Sam Harris, has been rather open about his sympathies for Dzogchen Buddhism and for Advaita Vedanta “Hindu” spirituality.)

Just as nobody speaks “language,” nobody practices “religion.” We speak Thai, Spanish or Russian. We’re Jews, Buddhists or Muslims.

A helpful way of thinking about religions comes from the Anglo-Austrian philosopher Ludwig Wittgenstein, and particularly from his collection of “Philosophical Investigations,” published posthumously in 1953.

There are classes of things that are connected to one another by an essential common feature. Triangles, for instance, despite their wide variety, are distinguished from other geometrical shapes by their having three angles totaling 180 degrees. All mammals have hair, mammary glands and self-regulating body temperatures.

Wittgenstein noticed, however, that there are other classes of things where, although we might assume that they’re linked by at least one essential common feature, that supposed common feature is exceedingly difficult, if not impossible, to find. He proposed that such things might, in many cases, be connected by a series of overlapping similarities while no single feature is actually common to all of them.

As a primary illustration, he chose games. They might all share some external quality such as “amusement,” but, otherwise, card games, board games, ball games, Olympic Games, gladiatorial games, video games and games such as “musical chairs” have very little in common. Yet we think of them all as populating a category that we call “games.”

Wittgenstein also thought of families, which is why he labeled his concept “family similarities” or “family resemblances.” (His original German term has been translated both ways.) Unique family characteristics such as build, features, eye color, gait and temperament “overlap and crisscross,” he observed. John may have inherited his father’s nose but not his height, whereas Bob has the father’s height without the nose and Jim has neither the altitude nor the nose, but, alone of the three sons, received Dad’s light blond hair, winning smile and dazzling charm.

If the broad category of “religion” is intended to contain such diverse phenomena as Zen, Pentecostalism, Shinto, Confucianism, Eastern Orthodox Christianity and Buddhist atheism, it must almost certainly be understood in the spirit of Wittgenstein’s “family resemblances,” because there’s little if anything internally that all religions share.

Outwardly, of course, they typically serve similar functions. They address matters of what the late Protestant theologian Paul Tillich called “ultimate concern.” They’re “life orientational,” prescribing moral conduct and behavior. They generally involve traditions, symbols and rituals. They often venerate saints of some kind, whether or not they use the actual word. Most are somehow involved with the “supernatural,” however conceived.

But the categories blur. American patriotism also has rituals, symbols and traditions. And Nazism certainly did. By displaying Lenin’s undecayed body in a mausoleum-shrine on Red Square, Soviet communism mimicked Eastern Orthodox saint veneration, and Marxism defined right and wrong for hundreds of millions. Some varieties of environmentalism, too, seem deeply “religious.”

It’s unnerving, for historians of religion, to admit that we can’t quite define our topic. But there you have it.

Daniel Peterson founded BYU's Middle Eastern Texts Initiative, chairs The Interpreter Foundation and blogs on Patheos. William Hamblin, also at BYU, has written several books and articles on pre-modern history. They speak only for themselves.