It's commonplace on the right to argue that it is "settled science" that human life begins at conception. People who argue otherwise are biology deniers, just like those who deny human-caused global warming. In a certain respect they have a point — conception surely leads to a human being — but they stand on fundamentally weaker ground when it comes to the strength of their scientific claim.
Mollie Hemingway points us to this list of definitions from biological textbooks and papers, all of which state that life begins when an egg is fertilized by a sperm. That is superficially similar to "ask the experts" studies that have been done about climate change, like the survey that found that 97 percent of practicing climate scientists accept anthropogenic climate change.
The problem is that there is no theory of life — there are only definitions. What, after all, constitutes life? This famous essay in Science gives seven key characteristics: an organized plan, improvisation, compartmentalization, energy, regeneration, adaptability, and seclusion. Others have proposed somewhat different schemes.
Their main purpose, however, is to include things that seem alive and exclude things that don't. No theory of life exists to explain why a bacteria is alive and viruses are not (or have a weird in-between status). Scientists simply place them in different categories based on definitional arguments.
Which is fine! There probably isn't any other way to do it. But like classifying species, this is about putting human labels on things, labels which tend to fray at the edges.
By contrast, consider the theory of evolution. Darwin took the vast species-labeling and -classifying exercise of Linnaeus and explained why species take the forms they do: because of the force of natural selection. And critically, like all scientific theories, it can be tested through experiment. That's why people call evolution the foundation of modern biology, since it went from being a descriptive field to a predictive one.
Similarly, climate change is based on the theory of quantum mechanics, which predicts that molecules that bend in a certain way (like carbon dioxide) will absorb and emit infrared light, trapping heat. Sure enough, tests in the lab confirm that this is indeed the case. Measurements of the Earth's temperature also confirm that increasing the atmospheric concentration of carbon dioxide (through human burning of fossil carbon) is associated with increasing temperature.
Thus to say that someone is a denier of climate science is to make a much stronger claim than one regarding the beginning of life: it is to say their beliefs may be easily disproved through experiment, not just that they disagree with a panel of scientists.
That's not to say that a lack of a scientific theory invalidates the anti-abortion position. It is to say that the debate over abortion is fundamentally philosophical and moral, not scientific.
For example, I'm fine with the idea that an abortion represents the end of life, which I distinguish from the idea of a human being. Indeed, I would even go further and bite the materialist bullet, which is to say that human identity itself is a fundamentally ephemeral result of the brain's self-representation. But that's a discussion for another time.
The point is that science has little to do with this argument. Science has enormous cultural legitimacy, so I can see why people want to claim it for their own position, or try to quibble with the definition of life. But when it comes to abortion, science simply doesn't have much to say.
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