Three ideas, planted by three thinkers more than 100 years ago, took root and spread into every realm of human activity during the 20th century.

Yet, as the century ends, large numbers of people still cannot live comfortably with those three ideas, which challenge the exalted position of humans in the universe and question the existence of God. But no matter how uncomfortable those ideas may feel, they certainly will persist well into the 21st century.

Sigmund Freud's theories concerning repression, the Oedipus complex and the death instinct have disintegrated over time. Yet, his conception of the unconscious stands and gains strength as scientists delve deeper into the mechanics of the brain.

Karl Marx's economic theories collapsed with the Soviet Union, but his ringing demands for social justice and national programs to help the masses remain audible.

Charles Darwin, the third thinker, has steadily gained in stature and influence since "The Origin of Species" was published in 1859. Not only does the concept of natural selection provide many people with a satisfying explanation for the bewildering diversity of life on Earth, it has been used to provide insight into other systems, from human mating behavior to the spread of ideas in society.

Perhaps the most practical way to greet the new millennium would be to come to terms with these three thinkers - and prepare for the influence they will have in the decades to come.

Understanding Freud "The Interpretation of Dreams" was published on Nov. 4, 1899, but the title page bears the date 1900, as though someone sensed the book belonged to the new century.

And it did. Although the existence of an unconscious mind was widely accepted by scientists of the 19th century, Freud popularized it in his book, declaring dreams to be the "royal road to the unconscious."

People embraced the notion. Suddenly the chaotic images that erupted from the brain during sleep were redefined as secret messages cleverly disguised to slip past the censor who normally blocks disturbing thoughts from crossing the threshold into awareness. Those who deciphered those messages, Freud argued, could expand their self-knowledge.

Some resisted Freud's ideas, but by mid-century Freudian thought had seeped so deeply into society that resistance was futile.

Today, however, virtually all of Freud's theories have been discarded. Nobel prize winner Sir Peter Medawar has even dismissed Freudian theory as "the most stupendous confidence trick of the 20th century."

But Freud still deserves credit for introducing the public to the unconscious, which may command an even larger portion of our mental life than Freud imagined, according to contemporary researchers. Exploring those dark recesses of the mind will be near the top of the agenda for brain scientists of the 21st century.

Marx's theories The ideas of Karl Marx became hopelessly entangled with communism, so when the Soviet Union fell apart, many Americans assumed his theories died as well.

However, while Marx's ideas did indeed inspire the Russian Revolution - one of the most significant events of the 20th century - the Soviet regime that followed in many ways violated Marx's deepest beliefs. Marx built his theory on the ancient Greek preoccupation with a just society. He challenged the notion that the privileged and the powerful are inherently superior to the rest of humanity. That may seem like common sense today, but in Marx's day members of the elite took it for granted that they were entitled to the power that flowed from their fortunes.

When Marx proclaimed this injustice, the masses balked at providing the labor that produced the wealth. Workers suddenly wanted a bigger piece of the pie. His ideas continue to threaten the rich and the powerful.

This is why Robert Heilbroner, author of "Marxism, For and Against," insists that Marx's work "remains immediate and indispensable, although many details of his explanation have been proved wrong."

Evolution of Darwin Charles Darwin's theory of evolution is so simple that an elementary school student can grasp it in two minutes: Since all the offspring of plants and animals differ a little, those best adapted to their environment are most likely to survive and pass their genes on to the next generation. This process of "natural selection" results in the "survival of the fittest" (a phrase introduced by biologist Herbert Spencer, but immediately embraced by Darwin).

The theory of evolution posed the most direct challenge to organized religion. If life on Earth arose through natural selection, then the biblical account of creation is false. But the theory is so compelling, and the fossil evidence is so persuasive, that no credible challenge has ever been mounted against it.

Yet, as the century ends, "creationists" can still persuade school boards that evolution is "just a theory," and should be taught side by side with the biblical account.

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