 One difficulty with eliminative induction is the source of the relevant alternative hypotheses, in this case reaction mechanisms. There is no algorithmic procedure for generating these mechanisms, and there is always the possibility that the correct mechanism has not been considered at all. This is a genuine problem, and we believe that it is the very issue that motivates chemists to turn towards falsification when thinking about mechanisms; all they can do is evaluate the plausible mechanisms that they have thought of. However, we see eliminative induction as a more plausible reflection of the epistemic situation of mechanistic chemists. This problem is not uncertainty about mechanisms compatible with experiments—chemists have evidence that weighs in favor of those. Rather, the problem is with unconceived alternatives. Structure offers one way to delineate such mechanistic possibilities: Hoffmann (1997, Chapter 29) provides a beautiful example of explicitly eliminative reasoning in his discussion of how H. Okabe and J. R. McNesby used isotopic labelling to eliminate two out of three possible mechanisms for the photolysis of ethane to ethylene. But this is an issue in all parts of science, not just mechanistic chemistry, and eliminative induction has always played a role in chemists’ reasoning about structure. How did van ’t Hoff argue for the tetrahedral carbon atom? He argued first that it was possible to account for the observed number and variety of the isomers of certain organic substances only by taking into account the arrangement of atoms in space. He then defended a tetrahedral geometry for the carbon atom by rejecting a square planar arrangement: if carbon’s geometry were square planar, there would be more isomers of substituted methane than are observed. Thus, for instance, disubstituted methane (of the form CH2X2) should have two separable isomers if it is square planar, whereas only one can be found. Assuming a tetrahedral arrangement, in contrast, would be in accord with the observed number of isomers (Brock 1992).