Sometimes it isn’t the heart of a champion, but a steely mindset. It’s remaining confident even when the losses pile up and the media counts you out and the fans begin to lose hope. It’s knowing you’ve done this before and you can do it again, even if conventional wisdom disagrees with you. It’s taking the last two months of the season and spitting on them, because you know there isn’t a single team that wants to see you come playoff time.

“The thing I’ve been impressed with is, it’s no alarm,’’ Jermaine O’Neal told the Boston Globe. “Nobody’s panicking. We understand if we play the way we’re supposed to play, we win.’’

The Boston Celtics had plenty of reason to doubt themselves, both this year and last. The basketball Grim Reaper had seemingly come for them, come to kill off the Big Three era with a rash of nagging injuries and old age, come to end championship-caliber basketball in Boston, at least for the time-being. But the Celtics had a trump card in their back pockets, the swagger of a champion, a shared confidence, or cockiness, an intangible stubbornness in their own abilities. They looked the Grim Reaper in his eyes, told him their time had not yet come, told him to visit the Cleveland Cavaliers and Orlando Magic instead.

You can crunch numbers for a long time, calculate the average margin of victory or the Hollinger rating or whatever else, but you can’t measure the belief these Celtics have in themselves, a belief which, I assume, comes from winning a championship. You can’t measure the value of looking at the guy in the locker room stall next to you and knowing you can rely on him, because you’ve done this before. When the Cavs drove over Boston with a Mack truck in Game 3 last season, at the TD Garden, it could have tore them apart. But pressure refines them, pressure makes them noble, and the Celtics didn’t lost to Cleveland again. When things went wrong for the Cavs, there was no past experience they could lean on, and thus adversity threw them into oncoming traffic.

“We’re going to have [adversity],” Doc Rivers told the Boston Globe. “It wasn’t an if. Be prepared for it. Embrace it, it’s a good thing. Enjoy the adversity. You find out who handles it well and who doesn’t, but even the ones who [don’t] at times, you hang in there with them, and eventually they’ll come through for you.’’

I’m not always a sucker for the intangible, but it’s hard to argue against the Celtics’ trust in themselves. You get the feeling they could lose 26 straight games (yet another Cavs reference) and still consider themselves the NBA’s best team. You get the feeling they could go down 0-3 in a series and still have no doubts they would come back. They’ve done this before and no matter what outside influences opine, no matter what advanced statistics or expert analysis tells them, think they can do it again. Sure, only five players remain from last season’s team, the same five who remain from 2008. But those five lead the rest. They instill a sense of calm during the furious storm of pressure.

For awhile, I thought Boston’s belief might be gone. I thought the Celtics might have too many new faces. I thought they might miss Kendrick Perkins for more than just his basketball talents. I thought the championship chemistry had been altered in a way that might be irretrievable. But it’s playoff time again and the Celtics are back to their old tricks, looking the Grim Reaper in his eyes and telling him it’s not their time.