After a speech in Romania last week, I was asked a question by a young member of the audience that, remarkably, I don’t remember ever having been asked: What is leftism?

The question seems so simple and so basic to understanding contemporary life that you would think it would be posed frequently — and if it weren’t, the reason would be that almost anyone who cares about society would already know how to answer it. But the question is rarely posed, and few know how to answer it.

“What is leftism?” is not at all the same as “What is liberalism?” Leftism and liberalism share almost no values. Indeed, perhaps the greatest tragedy of America today is that liberals do not vote their values. Leftists vote their values, and conservatives vote their values. But liberals vote for left-wing values, almost none of which they hold.

To cite but two examples: Liberals do not believe in racial segregation, whereas the left does: all-black dorms, all-black graduation exercises, different standards for blacks, etc. And liberals do not believe that males who say they are females should be allowed to compete in women’s sports; only leftists do.

If you ask people, “What is leftism?” most will respond by listing left-wing positions, not by defining it. People confuse identifying left-wing positions with knowing what leftism is. But naming left-wing positions is not the same as defining leftism.

To be fair, many people, including conservatives, cannot define conservatism either. In fact, the inability of millions of American conservatives to define conservatism is a major reason so many conservative parents don’t have conservative children. (See my two-part column, “Explaining Conservatism”)

Ironically, if conservatives had not defined conservatism but had only defined leftism, that alone would have ensured that many of their children would never have embraced left-wing views. The moment you understand leftism, almost everything that is happening in and to America at the present time becomes clear. And the moment that becomes clear, most people reject the Left.

What, then, is leftism?

Leftism is the attempt to destroy the past — every value and every institution, the good as well as what it regards as the bad.

That is why leftists, by definition, hate conservatism. Conservatism seeks to conserve the best from the past. The Left seeks to destroy the past, including the best.

The first of the modern left-wing revolutions, the French Revolution, quite consciously sought to destroy every major institution and value of French society — not just the monarchy but God, religion, the legal system, traditional notions of good and evil, the calendar, the old way of telling time, the old weights and measures, and even the names of the days of the week. In other words, the past. Just like the Left in contemporary America, the leftists of the French Revolution toppled statues — in their case, the statues of every king of France.

The next major left-wing revolution, the Russian, did the same. As the Soviet dissident joke went, “In the Soviet Union the future is known; it’s the past which is always changing.”

Now you can begin to understand leftism.

In a very real sense, today’s leftism began in the 1960s with the infamous clarion call, “Never trust anyone over 30.” That phrase meant nothing less than “value nothing from the past.” Precisely what all leftism has been about — from France under its revolutionaries to Russia under the Bolsheviks to China under Mao to America under the Left.

That is why, as I have said almost every day on my radio show for years, “The Left destroys everything it touches.”

Whatever its noble-sounding rhetoric, the Left stands for nothing and therefore builds nothing (other than state power). Aside from state power, it only destroys.

Leftism in music, art, sculpture, and architecture destroyed everything beautiful and noble that had been created over all the preceding centuries.

It is destroying the universities, the high schools, and the elementary schools.

It is destroying science. More and more medical schools, for example, no longer speak of “pregnant women” but of “birthing persons.” The American Medical Association has come out in opposition to listing the sex of newborns on their birth certificates; children, the AMA holds, will eventually decide whether they are male or female, neither or both.

Like the French revolutionaries, it has redefined moral categories. It has substituted class and racial categories for moral ones. Good and evil have been replaced by black and white, male and female, rich and poor.

It is destroying the ideal of the nuclear family — a married man and woman with children. The Left has made war on “heteronormativity” and has redefined marriage.

And most telling — even the French Revolution did not conceive of this break with the past — the Left is working to destroy the distinction between man and woman.

Days before the 2008 presidential election, then-candidate Barack Obama told a wildly cheering crowd, “We are five days away form fundamentally transforming the United States of America.”

That’s all the Left does: fundamentally transform. Destroying everything it touches is not a byproduct of leftism. It is its aim.

Note: The first sentence was edited by Mr. Prager after publication for better clarity.

        Dennis Prager is a nationally syndicated radio talk-show host and columnist. His commentary on Deuteronomy, the third volume of “The Rational Bible,” his five-volume commentary on the first five books of the Bible, was published in October. He is the co-founder of Prager University and may be contacted at dennisprager.com.