By David Deming

President Barack Obama has said that the science of global warming is "beyond dispute" and therefore settled. This is the justification for the imposition of a carbon cap-and-trade system that will cost $2 trillion.

Unfortunately, Mr. Obama does not understand science. "Settled science" is an oxymoron, and anyone who characterizes science as "settled" or "indisputable" is ignorant not only of science, but also of history and philosophy.

Aristotle, who lived and wrote in the 4th century BC, was one of the greatest geniuses the world has ever known. He invented the discipline of logic, and founded the sciences of ecology and biology. Aristotle's physics was accepted as correct for nearly two thousand years. In 1534, faculty at the University of Paris officially asserted that the works of Aristotle were "the standard and basis of all philosophic enquiry."

Aristotle taught that heavy objects fall faster than light ones. Over the centuries, a few unreasonable persons expressed skeptical concerns. But the consensus was that the physics of motion was described by Aristotle's dicta. The science was settled.

Around the year 1591, an irascible young instructor at the University of Pisa demonstrated that Aristotle was wrong. He climbed to the top of the tower of Pisa and dropped cannon balls of unequal weight that hit the ground simultaneously. Aristotelian professors on the faculty were embarrassed. The University administration responded by not renewing Galileo's contract, thus ridding themselves of a troublemaker who challenged the accepted consensus.

Of course, Galileo is better remembered today for clashing with the Catholic Church over the issue of whether or not the Earth was at the center of the universe. An Earth-centered cosmology was first proposed by the Greek philosopher, Eudoxus, in the 4th century BC. About a hundred years later, an upstart named Aristarchus suggested that the Earth revolved around the Sun. Aristarchus' system never proved popular, and he was criticized for being impious.

The Earth-centered system was finalized by Claudius Ptolemy in the second century AD, and remained unchallenged until the sixteenth century. Everyone knew that the science of astronomy had been settled "beyond dispute." When Galileo insisted that the Earth revolved around the Sun, he was castigated by the Church for advocating an idea that was not only heretical, but also "foolish and absurd in philosophy."

Late in the seventeenth century, Isaac Newton demonstrated definitively that Aristotle's physics was incorrect. He proposed the Law of Universal Gravitation, and explained how the planets move around the Sun in elliptical orbits. Newton is still regarded as the greatest scientist who ever lived. He settled the science of motion in such a conclusive way that his system was referred to as an "invincible edifice." But the edifice crumbled early in the twentieth century when Einstein showed that Newtonian physics breaks down as the speed of light is approached.

Near the beginning of the nineteenth century, the Neptunian School of geology taught that all rocks had formed by crystallization from a now-vanished universal ocean. Although the evidence falsifying this theory was both plain and abundant, Neptunists interpreted every observation as supportive of their hypothesis. Blinded by an immoderate zeal, they selected and magnified any fact in accordance with their theory, while neglecting those that tended to disprove it. Robert Jameson characterized the evidence supporting Neptunism as "incontrovertible." But the theory collapsed in a few decades, and today is recognized as an artifact of inexhaustible human folly.

Mr. Obama, a lawyer and politician, would now have us believe that the process of history has stopped. For the first time, scientific knowledge is not provisional and subject to revision, but final and settled. Skepticism, which has been the spur to all innovation and human progress, is unacceptable and must be condemned.

But in fact, it is our awareness of what we do not know that determines our scientific level. Socrates was the wisest man, not because he knew more than others, but because he was the only one to recognize that he did not know. Knowledge begins with skepticism and ends with conceit.