Cite as: 529 U. S. 513 (2000)
Opinion of the Court
competent to testify unless corroborated by another witness). Plainly, the imagined rule does not mean that Fenwick's case is not an example of an ex post facto law. But if that is so, why should it be any different for Article 38.07? Just as we can imagine a witness competency rule that would operate similarly to the statute in Fenwick's case, the above argument imagines a witness competency rule that operates similarly to Article 38.07. If the former does not change our view of the law in Fenwick's case, why should the latter change our view in the present circumstances?
Moreover, the argument fails to account for what Calder's fourth category actually says, and tells only half the story of what a witness competency rule does. As for what Calder says, the fourth category applies to "[e]very law that alters the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offence, in order to convict the offender." 3 Dall., at 390 (emphasis deleted). The last six words are crucial. The relevant question is whether the law affects the quantum of evidence required to convict; a witness competency rule that (in certain instances at least) has the practical effect of telling us what evidence would result in acquittal does not really speak to Calder's fourth category.
As for relating only half the story, the dissent's argument rests on the assertion that sometimes a witness competency rule will result in acquittals in the same instances in which Article 38.07 would also demand an acquittal. That may be conceded, but it is only half the story—and, as just noted, not the most relevant half. The other half concerns what a witness competency rule has to say about the evidence "required . . . in order to convict the offender." The answer is, nothing at all. As mentioned earlier, see supra, at 546- 547, prosecutors may satisfy all the requirements of any number of witness competency rules, but this says absolutely nothing about whether they have introduced a quantum of
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