Joe Biden, who was President Obama’s vice president for eight years, said Obama used to often ask him during his second term what would motivate him to run for president.

Rev. Al Sharpton raised the same question during a lengthy sit-down with Biden that aired this morning on MSNBC’s Politics Nation.

“I’m really hoping some other folks step up,” Biden demurred. “I think we have some really good people.” When Sharpton pressed it, though, he clarified that he was not ruling anything out, and would decide by the end of 2018.

After the longtime senator and VP had to abandon a potential run against Hillary Clinton in the 2016 race because of the fatal illness of his son, Beau, many Democrats wondered what might have been had Biden entered the ring. He told Sharpton that attending to his extended family’s ongoing recovery from Beau Biden’s death would be the main obstacle to him running.

“I’ve got to be able to look in the mirror,” Biden explained. “If I walk away I’ve got to know that I’m not walking away because I’m afraid or I’m worried about losing or I just don’t want to take on the responsibility. … For me, it’s selfish but it’s about putting my family back together.”

Recent polls that ranked him at the top of the heap among projected candidates “don’t mean a thing if you’re not in the ring,” Biden said. Asked by Sharpton if a faceoff with Donald Trump — and the two have already resorted to physical threats against each other — would be a “battle for the soul of America,” Biden said only, “We are fundamentally different.”