HARRISBURG, Pa. -- The merits of teaching "intelligent design" as a basis for human development are being argued in a Pennsylvania courtroom.

In opening statements Monday, a lawyer for eight families who contend it's unconstitutional argued it's a religious theory with no scientific backing.

Eric Rothschild told a federal judge in Harrisburg that the Dover Area School District incorporated the viewpoint in its curriculum with no thought "about its scientific validity."

But, the school district's attorney countered that having ninth-grade students read a statement on intelligent design "embodies the essence of liberal education."

Patrick Gillen of the Thomas More Law Center in Ann Arbor, Mich., said, "This case is about free inquiry in education, not about a religious agenda."

Intelligent design holds that Charles Darwin's theory of natural selection cannot fully explain the origin of life or the emergence of highly complex life forms. It implies that life on Earth was the product of an intelligent force.

Critics say intelligent design is merely creationism camouflaged in scientific language, and it does not belong in a science curriculum.

After opening statements, the first expert witness, Prof. Kenneth Miller of Brown University, took the stand.

"Intelligent design is not science. It cannot be construed as scientific theory," Miller said.

About 75 spectators crowded the courtroom of federal District Judge John Jones for the start of the non-jury trial.

The trial marks the first constitutional test of intelligent design, which the Dover School Board voted to include in school curriculum in November 2004.