(Reuters) Lawrence Hurley Andrew Chung October 7 2019 10:42 AM WASHINGTON - The U.S. Supreme Court on Monday debated the insanity defense in criminal prosecutions and delivered a potential setback to gun control advocates as it opened its new term on Monday with a flurry of activity on a day when its longest-serving justice Clarence Thomas was absent due to illness. The conservative-majority court returned after a three-month break to launch its new nine-month term with arguments in a trio of cases. The court hears the terms first major case on Tuesday on whether a landmark federal anti-discrimination law that bars sex discrimination in the workplace protects gay and transgender employees. In remarks from the bench before the first argument Chief Justice John Roberts said Thomas was indisposed" due to illness but would participate in deciding the cases argued on Monday. A court spokeswoman said Thomas 71 likely has the flu. One of the nine-member courts most conservative justices Thomas has served since 1991. The court said it would not cancel its scheduled Dec. 2 arguments as requested by New York City in a lawsuit by gun owners and the states National Rifle Association affiliate challenging restrictions on handgun owners transporting firearms outside the home. The city had asked the justices to ditch the arguments because its measure was recently amended meaning there was no longer any reason to hear the dispute. Hearing the case could enable the courts conservative majority to rule against a gun control measure although the justices also still could dismiss it after the arguments. During Mondays first argument the justices wrestled with the insanity defense which bars holding criminally responsible mentally impaired defendants who do not know right from wrong. They seemed divided over a Kansas law enacted in 1996 that prevents defendants from arguing insanity. The law lets defendants argue that due to mental illness they could not have intended to commit the crime.