This article, written for a symposium on "Guilt v. Guiltiness: Are the Right Rules for Trying Factual Innocence Inevitably the Wrong Rules for Trying Culpability?," argues that the definition of expertise in the criminal justice system, derived in the federal courts and in most states from Daubert v. Merrell Dow Pharmaceuticals Co., should vary depending on whether the issue involved is past mental state or past conduct. While expert psychological testimony about past acts ought to be based on scientifically verifiable assertions, expert psychological testimony about subjective mental states relevant to criminal responsibility need not meet the same threshold. This stance derives from the interplay between what I call the "right to voice" and a necessity rationale. The right to voice derives from the Constitution, which can be read to give criminal defendants a break whenever they want to present expert testimony. But respect for Daubert and scientific epistemology counsels that this bow to the defense should occur only when a scientific approach is not possible. Such an approach *is* futile with respect to opinion testimony about past mental state, so a relaxed evidentiary standard should govern there. However, when, as with past act testimony, the necessity argument is weak, Daubert should apply with full force.