When Tom Brady’s appeal hearing kicks off Tuesday, key arguments will be about who ordered his four-game suspension and whether science supports the league’s findings about deflated footballs.

The NFL says Commissioner Roger Goodell authorized the discipline that was imposed by league executive Troy Vincent, who signed the letters sent to Brady and the New England Patriots informing them of the penalties. The NFL Players Association challenged Vincent’s power to issue punishment, citing Article 46 of the league’s collective bargaining agreement.

“You have no authority to impose discipline on Mr. Brady under the CBA, and such discipline must therefore be set aside,” union attorney Tom DePaso wrote to Vincent on May 14. “The CBA grants the Commissioner — and only the Commissioner — the authority to impose conduct detrimental discipline on players.”

Goodell dismissed the union’s claim when he declined to recuse himself from hearing the appeal on June 23.

“I did not delegate my disciplinary authority to Mr. Vincent; I concurred in his recommendation and authorized him to communicate to Mr. Brady the discipline imposed under my authority as Commissioner,” Goodell said in his letter to the union on June 2. “The identity of the person who signed the disciplinary letter is irrelevant.”

Brady was suspended four games and the Patriots were fined $1 million and docked a pair of draft picks after investigator Ted Wells found that the Super Bowl champions illegally used under-inflated footballs in the AFC title game.

Vincent has issued several fines and penalties for various infractions since replacing Ray Anderson as the NFL’s executive vice president of football operations in March 2014.

He suspended former Redskins safety Brandon Meriweather two games last September for a helmet-to-helmet hit on a receiver. In several other cases, he fined teams or punished team officials for violating rules.

The NFLPA didn’t question Vincent’s authority in those incidents but it only gets involved when players are disciplined.

Goodell issued punishments to Greg Hardy, Adrian Peterson and Ray Rice in recent, high-profile cases involving players violating the league’s personal conduct policy. The league doesn’t consider Brady’s case similar because it involved rules of the game.

Scientific arguments also will be a major part of Brady’s defense led by attorney Jeffrey Kessler. Brady’s lawyers are expected to try to shoot down the findings of an independent firm that was hired to provide scientific analysis of the air pressure inside the footballs used by the Patriots and Colts during the AFC championship game.

Brady’s side will also argue that:

— The evidence collected in the Wells report doesn’t prove Brady violated any NFL rules.

— The punishment is harsher than for similar violations.

While Brady is fighting his punishment, Patriots owner Robert Kraft declined to appeal the team’s penalty, though he defended his franchise player and denied any wrongdoing by team employees.

Goodell said he will give Brady an opportunity to introduce new evidence next week. If Brady’s suspension isn’t overturned, the battle could end up going to court. For now, it’s up to Goodell.

When Tom Brady’s appeal hearing kicks off Tuesday, key arguments will be about who ordered his four-game suspension and whether science supports the league’s findings about deflated footballs.

The NFL says Commissioner Roger Goodell authorized the discipline that was imposed by league executive Troy Vincent, who signed the letters sent to Brady and the New England Patriots informing them of the penalties. The NFL Players Association challenged Vincent’s power to issue punishment, citing Article 46 of the league’s collective bargaining agreement.

“You have no authority to impose discipline on Mr. Brady under the CBA, and such discipline must therefore be set aside,” union attorney Tom DePaso wrote to Vincent on May 14. “The CBA grants the Commissioner — and only the Commissioner — the authority to impose conduct detrimental discipline on players.”

Goodell dismissed the union’s claim when he declined to recuse himself from hearing the appeal on June 23.

“I did not delegate my disciplinary authority to Mr. Vincent; I concurred in his recommendation and authorized him to communicate to Mr. Brady the discipline imposed under my authority as Commissioner,” Goodell said in his letter to the union on June 2. “The identity of the person who signed the disciplinary letter is irrelevant.”

Brady was suspended four games and the Patriots were fined $1 million and docked a pair of draft picks after investigator Ted Wells found that the Super Bowl champions illegally used under-inflated footballs in the AFC title game.

Vincent has issued several fines and penalties for various infractions since replacing Ray Anderson as the NFL’s executive vice president of football operations in March 2014.

He suspended former Redskins safety Brandon Meriweather two games last September for a helmet-to-helmet hit on a receiver. In several other cases, he fined teams or punished team officials for violating rules.

The NFLPA didn’t question Vincent’s authority in those incidents but it only gets involved when players are disciplined.

Goodell issued punishments to Greg Hardy, Adrian Peterson and Ray Rice in recent, high-profile cases involving players violating the league’s personal conduct policy. The league doesn’t consider Brady’s case similar because it involved rules of the game.

Scientific arguments also will be a major part of Brady’s defense led by attorney Jeffrey Kessler. Brady’s lawyers are expected to try to shoot down the findings of an independent firm that was hired to provide scientific analysis of the air pressure inside the footballs used by the Patriots and Colts during the AFC championship game.

Brady’s side will also argue that:

— The evidence collected in the Wells report doesn’t prove Brady violated any NFL rules.

— The punishment is harsher than for similar violations.

While Brady is fighting his punishment, Patriots owner Robert Kraft declined to appeal the team’s penalty, though he defended his franchise player and denied any wrongdoing by team employees.

Goodell said he will give Brady an opportunity to introduce new evidence next week. If Brady’s suspension isn’t overturned, the battle could end up going to court. For now, it’s up to Goodell.