TENNESSEE LAST WEEK enacted a law that prohibits the punishment of teachers for discussing the “scientific strengths and scientific weaknesses” of global warming, evolution and the chemical origins of life.

The bill itself has more weaknesses than strengths.

Before the law, Tennessee science teachers hardly needed to fear admitting that there are some questions for which science does not offer clear answers. There was nothing to keep them from discussing scientific uncertainties inherent to particular fields: just how sensitive the climate is to carbon emissions, for example.

Nor did instructors teaching world religions or philosophy in Tennessee have any reason to avoid neutrally discussing alternative narratives about the origins of the universe, the development of species or the future of the planet. Exposure to foundational critiques of modernity — particularly those concerning when and how people really know something to be true and the role of belief in society — is essential to developing critical thinking skills. None of that should be controversial, even among the most ardent of empiricists.

What, then, does the state’s new law accomplish? Not a thing, as Tennessee Gov. Bill Haslam (R), who allowed the bill to become law without his signature, admitted. Rather than removing some kind of official hostility to critical thought in Tennessee’s curriculum, it seems designed to encourage teachers who would introduce pseudo-scientific criticisms inspired by religion or ideology into descriptions of the current state of evolution or climate science.

Even if that wasn’t the intent of the law, that’s the most likely consequence, if any, of its passage. Merely emphasizing the existence of notable “scientific weaknesses” exaggerates the uncertainty among scientists about these theories. That the state legislature has gone out of its way to warn administrators not to touch teachers can only discourage them from pushing back against wayward instructors.

Mr. Haslam rightly expressed concern that the law would fog state education policy, introducing a lack of clarity that is easy to interpret as pretext for doing more than its words superficially suggest. Unfortunately, the governor also didn’t muster the courage to veto the bill. Such a stand would have been significant, if only symbolically: The legislature had the votes to override. Other governors who find themselves in Mr. Haslam’s position should fight back harder.