The foregoing evidence, which is squarely contradicted by the defendants, raises a credibility issue. While summary judgment may be based on affidavits, conflicts of credibility should not be resolved on a hearing on the motion for summary judgment unless the opponent’s evidence is “too incredible to be believed by reasonable minds.” 6 J. Moore, Moore’s Federal Practice 11 56.15(4), at 56-524 (2d ed. 1976); see generally id. at 56-512.3-56-530. When there are conflicting statements by witnesses, cross-examination may be determinative. As the Second Circuit stated in one of the seminal summary judgment cases, “we cannot very well overestimate the importance of having the witness examined and cross-examined in presence of the court and jury.” Arnstein v. Porter, 154 F.2d 464, 470 (2d Cir.1946) (citation omitted). See also Poller v. CBS, 368 U.S. 464, 473, 82 S.Ct. 486, 491, 7 L.Ed.2d 458 (1962).
In cases where substantive liability depends on the reasonableness of the defendants in making a probable cause determination, there is often an issue of credibility. This is particularly true because defendants bear the burden at trial of prov-ing the defense of good faith and probable cause, see Pierson v. Ray, 386 U.S. 547, 556-57, 87 S.Ct. 1213, 1218-19, 18 L.Ed.2d 288 (1967), which would ordinarily require their own testimony. Thus, this court stat-. ed in Patzig v. O’Neil that the reasonableness of defendants’ probable-cause determination is for the jury. 577 F.2d at 848. See also B.C.R. Transport Co. v. Fontaine, 727 F.2d 7, 10 (1st Cir.1984).
We express no views as to the persuasiveness of Losch’s evidence. The district court was not persuaded, but it is the jury and not the court who is the ultimate fact finder. We conclude only that Losch’s evidence is not incredible and should have survived a motion for summary judgment unless the defendants were entitled to prevail on some other basis.
III.
Qualified Immunity
The individual defendants have invoked the defense of qualified immunity, under which “government officials performing discretionary functions generally are shielded from liability for eivil damages insofar as their conduct does not violate clearly established statutory or constitutional rights of which a reasonable person would have known.” Harlow v. Fitzgerald, 457 U.S. 800, 818, 102 S.Ct. 2727, 2738, 73 L.Ed.2d 396 (1982). While the defendant has the burden of pleading and proving qualified immunity, id. at 815, 102 S.Ct. at 2737, the objective standard as announced in Harlow allows a court to cut short the inquiry into a defendant’s state of mind and to grant summary judgment in “insubstantial” claims. Id. at 818, 102 S.Ct. at 2738; Forsyth v. Kleindienst, 729 F.2d 267, 273 (3d Cir.1984).
Qualified immunity protects a defendant who failed to observe a right or a legal standard that was not clearly established when the defendant acted. Thus, the Court held in Procunier v. Navarette, 434 U.S. 555, 562-65, 98 S.Ct. 855, 859-61, 55 L.Ed.2d 24 (1978), for example, that prison officials were not liable for interfering with an inmate’s mail because the interference occurred before there was any definite First Amendment protection for prisoners’ mail privileges. On the other hand, as the Harlow Court concluded, “[i]f the law was clearly established, the immunity defense ordinarily should fail,” unless the defendant “claims extraordinary circumstances and can prove that he neither knew nor should have known the relevant legal standard.” 457 U.S. at 818-19, 102 S.Ct. at 2739.
In determining that the individual defendants were entitled to qualified immunity because the reach of the Pennsylvania criminal statutes may not have been clear, the district court misconceived the focus of the qualified immunity inquiry. As it applies here, the inquiry must focus