Justice Antonin Scalia, in the 1992 Supreme Court case of United States v. Williams, explained what the role of a grand jury has been for hundreds of years.
It is the grand jury’s function not ‘to enquire … upon what foundation [the charge may be] denied,’ or otherwise to try the suspect’s defenses, but only to examine ‘upon what foundation [the charge] is made’ by the prosecutor. Respublica v. Shaffer, 1 Dall. 236 (O. T. Phila. 1788); see also F. Wharton, Criminal Pleading and Practice § 360, pp. 248-249 (8th ed. 1880). As a consequence, neither in this country nor in England has the suspect under investigation by the grand jury ever been thought to have a right to testify or to have exculpatory evidence presented.This passage was first highlighted by attorney Ian Samuel, a former clerk to Justice Scalia.