On May 7, 1959, the celebrated British novelist and critic, C.P. Snow, gave a lecture at Cambridge University, entitled “The Two Cultures,” in which he described a complete breakdown in communication between the scientific world and the humanities. Not only were most scientists of his day largely unexposed to the most important ideas and values found in literature and philosophy, he argued, but the rest of the otherwise well-educated population was profoundly ignorant of even the most elementary scientific principles. Unless this disconnect is addressed, he warned, civilization would suffer.

A half century later, the gulf between the scientists and the rest of us appears wider than ever, and, judging from the impassioned arguments put forward by Robert P. George and Christopher Tollefsen in “Embryo” (Doubleday, 256 pages, $23.95), what is at stake is the very definition of life itself.

This book is likely to make a lot of people crazy: It is a radical, even audacious, assault on the emerging technologies that would harvest living human embryos for medical research purposes. It is absolutist in its claim that human life begins at fertilization, when the male and female gamete, each bearing 23 complementary chromosomes, combine to create the single-cell zygote that will implant itself in the uterus and, in due course, become a man or woman. The argument’s implications, not only for embryonic research but for abortion and some forms of contraception, are obvious: If it’s human, you shouldn’t kill it. That the argument relies on no sectarian religious tenet will only further aggravate those who disagree — it is much easier, these days, to dismiss religious scruple than scientific fact and logic.

Facts are notoriously stubborn things, and the facts about fetal development have been on a collision course with “settled” law for quite some time. It was no great effort to dismiss the notion of humanity in the womb when there were no sonograms to show expectant parents the fingers and the toes, and it wasn’t all that long ago when people thought “the quickening” movement in the womb was the beginning of life. Now we can see for ourselves the continuum that is human gestation. Even Hollywood has figured this one out, dribbling out a series of recent hits that imply abortion is far more morally dangerous than its treatment in earlier movies would imply.

What sets this book apart is its detailed analysis, and its compelling refutation of the arguments for treating embryos as nothing more than a mass of cells suitable for research purposes. Mr. George, a professor of jurisprudence at Princeton, and Mr. Tollefsen, the director of the graduate department of philosophy at the University of South Carolina, begin with a careful examination of the science of embryology, which clearly shows that once fertilization occurs the new life created has “a single, unified and self-integrated biological system and a developmental trajectory.” In short, like a fetus, an infant, a child, and an adolescent, the embryo only needs a supportive environment to become an adult human being.

Messrs. George and Tollefsen then review the philosophical underpinnings of pro-research arguments, ranging from René Descartes’s theory of sentience (“I think, therefore I am”) to the utilitarianism of Jeremy Bentham and the “greater good” theories of John Stuart Mill. It appears that the most salient arguments for withholding moral respect and, thus, protection from embryonic life are the absence of a developed brain and the promise of treating terrible diseases with the stem cells harvested from embryos.

But once the embryo is defined as human — as the science of embryology clearly defines it — the sentience argument falls short. Why not also harvest organs from the severely retarded or the comatose? The history of assigning value to individual human lives based on perceptions of inferiority or inconvenience has not been a pretty one, and the “greater good” argument is undeniably stronger, provided that the extravagant claims made for embryonic stem cells are not exaggerated. (The authors and many others believe the claims are, in fact, exaggerated.) But it, too, falters when one considers the history of reckless medical experimentation — the notorious Tuskegee syphilis trials, for instance, or the radiation tests performed on the unsuspecting by the military.

In the end, however, the argument really boils down to the intrinsic value of human life. According to the “settled” law, a fetus has a value if it is wanted by its mother or, in some cases, if it can survive apart from her. A fetus lost in a car wreck represents a tort claim on an insurance company. But a fetus lost to an abortion has no more legal standing than a baked potato.

This is a difficult book to read. It is replete with scientific and philosophic jargon and its sentence structure is occasionally unfortunate. And while they are careful to keep religion out of their argument, both authors are known to be practicing Catholics, which will certainly be held against them, and their assertions, in a number of circles. Still, their book should stand as a serious challenge to the conventional wisdom on embryo research. And in bringing the wonders and dangers animating the world of science under closer scrutiny, they perform a valuable public service.

Mr. Willcox, a former editor in chief of Reader’s Digest, lives in Ridgefield, Conn.