 Experiments from molecular biology have also figured into philosophical discussions about the possibility of “crucial experiments”. An experiment is taken to be a crucial experiment if it is devised so as to result in the confirmation of one hypothesis by way of refuting other competing hypotheses. But the very idea of a crucial experiment, Pierre Duhem pointed out, assumes that the set of known competing hypotheses contains all possible explanations of a given phenomenon such that the refutation of all but one of the hypotheses deductively ensures the confirmation of the hypothesis left standing. However, if there are in fact unknown alternatives that weren’t considered in the set of competing hypotheses, Duhem warned, then the refutation cannot guarantee the confirmation of the last hypothesis standing (also see the entry on Pierre Duhem). (Duhem actually raised two problems for crucial experiments—the problem mentioned above, as well as the problem of auxiliary assumptions, which any hypothesis brings with it; for reasons of space, we will only discuss the former here.) Marcel Weber (2009) has utilized a famous experiment from molecular biology to offer a different vision of how crucial experiments work. After Watson and Crick discovered the double helical structure of DNA, molecular biologists turned their attention to how that macromolecule could be replicated (see  Section 1.2  above). The focus was in part on the fact that the DNA was twisted together in a helix, and so the challenge was figuring out what process could unwind and replicate that complexly wound molecule. Three competing hypotheses emerged, each with their own prediction about the extent to which newly replicated DNA double helices contained old DNA strands versus newly synthesized material: semi-conservative replication, conservative replication, and dispersive replication. Matthew Meselson and Frank Stahl, at Cal Tech, devised a method for testing among these competing hypotheses (see The Semi-Conservative Replication of DNA in  Other Internet Resources).  They grew E.coli on a medium that contained a unique, heavier form of nitrogen which could be taken up into the E.coli DNA; then they transferred that E.coli with heavy DNA to a medium with the more common, lighter form of nitrogen and let the E.coli replicate there. By then taking regular samples of the replicating E.coli and examining the DNA of the subsequent generations, they determined the extent to which the new DNA was a mix of old DNA strands or newly synthesized material. The result was a clear victory for semi-conservative replication, and the Meselson-Stahl experiment became known as the “most beautiful experiment in biology” (Meselson and Stahl 1958; Holmes 2001). Weber argues that we should understand the quick uptake of Meselson and Stahl’s experimental result as an instance of inference to the best explanation (as opposed to Duhem’s deductive characterization). Meselson and Stahl, Weber claims, took the physiological mechanism of DNA replication and then embedded it in an “experimental mechanism”; that experimental mechanism then generated the data pattern of heavy-vs-light DNA in subsequent E.coli generations. Moreover, any hypothesis of DNA replication had to satisfy mechanistic constraints imposed by what was already known about the physiological mechanism—that DNA was a double helix, and that the sequence of nucleotides in the DNA needed to be preserved in subsequent generations. So Duhem’s concern about unknown alternatives was alleviated because known mechanistic constraints limited the set of possible hypotheses that could generate the phenomenon. On Weber’s reading, the mechanistic constraints culled the set of possible hypotheses for DNA replication to semi-conservative replication, conservative replication, and dispersive replication; then, among that set, Meselson and Stahl devised an experimental mechanism such that semi-conservative replication was the best explanation of the data pattern they found. (For a critique, see Baetu 2017.)