In Witherspoon v. Illinois, 391 U.S. 510,(1968), the Court stated that only those prospective jurors could be excluded for cause who made it "unmistakably clear ... that they would automatically vote against the imposition of capital punishment without regard to any evidence that might be developed at the trial of the case before them" or whose "attitude toward the death penalty would prevent them from making an impartial decision as to the defendant's guilt." The Court concluded that "the most that can be demanded of a venireman in this regard is that he be willing to consider all of the penalties provided by state law and that he not be irrevocably committed, before the trial has begun, to vote against the penalty of death regardless of the facts and circumstances that might emerge in the course of the proceedings." (Id., at 522, n 21; emphasis supplied.) Seventeen years later in Wainwright v. Witt (supra, at 424, quoting Adams v. Texas, 448 US 38, 45 [1980]), the Court held that the proper standard by which a prospective juror may be challenged and excluded for cause is "whether the juror's views would 'prevent or substantially impair the performance of his duties as a juror in accordance with his instructions and his oath.' "The Court thereby clarified its decision in Witherspoon, dispensing with Witherspoon's reference to "automatic" decision making and the need to prove a juror's bias with "unmistakable clarity" (Witt, at 424). This, the Court reasoned, "is because determinations of juror [views] cannot be reduced to question-and-answer sessions which obtain results in the manner of a catechism" (id.). However, the Court did not explicitly overrule Witherspoon, noting that the standard found therein was dicta embodied in footnotes rather than in the main body of text (id., at 422).