You might wonder if there's any point in even trying to define liberalism. Efforts to do so seem bound to fail. From the start, its meaning has been elusive and in flux. Today, no right-thinking person is against "liberal democracy," and we mostly take "liberal capitalism" for granted — yet conservative Americans use "liberal" as a term of abuse and many left-leaning Americans would rather be called "progressives."

It's tempting to say that "liberalism" no longer means anything. This would be wrong, according to Edmund Fawcett's new book, "Liberalism: The Life of an Idea." Fawcett, a former colleague at the Economist, examines liberalism through time not as a fixed coherent ideology but as "a practice guided by four loose ideas": acceptance of conflict, resistance to power, belief in progress and civic respect. This is a novel approach, and it turns out to be very rewarding. Fawcett has written a marvelous book.

He steers the reader through a fascinating historical survey of liberalism's leading practitioners — meaning the thinkers and politicians who were guided, to a greater or lesser degree, by the four ideas. He ranges far beyond the usual cast of British and American principals. Indeed, his erudition would be daunting if he didn't write with such verve. "Liberalism" isn't an easy read, but it's a pleasure.

Fawcett's organizing bundle of four beliefs or attitudes is surprising at first sight because of what it seems to leave out. What about liberty? That's where most accounts of liberalism start, and where a lot of them finish.

Fawcett explains why liberty doesn't get you far in explicating liberalism: "Just about every modern rival to liberalism has claimed to stand somehow on the side of liberty." (The Nazi Party's charter program of 1920, he notes, called for "Germany's rebirth in the German spirit of German Liberty.") It doesn't help that liberals can't agree on what "liberty" is: Would that be negative liberty ("freedom from"), positive liberty ("freedom to") or something else? The notion that liberty is the foundational goal from which liberals derive all the rest doesn't wash, either. (For many liberals, equality counts at least as much.)

What, then, do liberals want when they say they want liberty? One thing is resistance to power, the second of Fawcett's guiding ideas: not just political power, but economic and social power as well. Liberalism expects that power tends toward tyranny unless checked. Another thing liberals want when they say they want liberty is respect for people in their own right — also one of Fawcett's guiding ideas. "Once embraced democratically, respect for people as such forbade power from excluding anyone from the circle of liberal protection." He calls this "the democratic seed in an otherwise undemocratic creed."

The practice of liberalism was also guided by the idea that social harmony was impossible. Goals were bound to conflict, but this disharmony could be channeled into competition, argument and exchange, which would make it a strength, not a weakness. Finally, liberal optimism and the liberal reading of history inclined practitioners to the view that change was both inevitable and, on the whole, for the better.

The four ideas are complementary in some respects and in contention in others. Liberals have always argued, and probably always will, about the weight that each idea should be given in relation to the rest, but it's been a strength of the practice of liberalism that no one idea has been permanently granted pre-eminence. Of course, one of the guiding ideas — that intellectual conflict is inevitable — predicts that very outcome. Liberals exemplify it by arguing with each other, as well as with nonliberals and anti-liberals.

Fawcett's four guiding ideas lend what could have been a sprawl of biographical sketches and wide-ranging intellectual history a satisfying and intelligible shape. What's better, the more you think about them, the more sense they make.

In particular, they help you to see what liberalism isn't. As Fawcett says, "The twentieth century was generous to liberalism with two defining Others, fascism and communism."

Conflict over goals? Both insisted on a false unity. Resistance to power? Fascism exalted the power of nation and race, communism the power of the proletariat. Belief in progress? Sure, but they defined it in terms that excluded outsiders. Civic respect? If you opposed the program, you were worthless. Communism "was an extremism of hope, fascism an extremism of hate. They were nevertheless alike enough on those four counts to provide liberalism with a captivating image of itself in negative."

Sadly, however, it isn't true to say, "We're all liberals now." There is no lack of rival systems: state capitalism, militant Islam, elected autocracy, outright dictatorship. All of those, in different ways, reject one or more of the guiding ideas of liberalism. And anti-liberal strains of thinking are all too apparent in the United States and other liberal societies. Nonetheless, there is such a thing as liberal society, and liberalism really is a big enough tent to accommodate progressive liberals as well as tea-party liberals.

Family quarrels can be bitter — but they're still family quarrels.

Clive Crook is a Bloomberg View columnist and a member of the Bloomberg View editorial board.