The Boston Celtics are accomplished and they are proud, and occasionally those traits morph them into cocky and disrespectful. Their performance in Game 4 — when they could have, and should have, opened up a 3-1 series lead — brought the latter traits into the forefront, but the former characteristics provide a basis to believe the Celtics will not be unraveled by their 18 minutes of dysfuntion.

The Big Three-era Celtics had never blown an 18-point postseason lead before Friday night. The Sixers, for 24 minutes, were everything that bothers the Celtics. They were athletic and long and attacking and they would not stop pounding the glass, nor would they stop forcing turnovers and getting easy looks in transition.  The Sixers spent the final two quarters of Friday’s game giving a clinic in how to beat the Celtics — limit Rondo’s transition opportunities, body the Celtics in the paint, be as physical as possible with Kevin Garnett, and use dribble penetration to break down one of the NBA’s fiercest defenses. The Sixers were much, much better than Boston during the latter portion of Game 4.

But the Celtics are accomplished and they are proud, and if you think they lost any confidence as a result of the battering, you probably haven’t paid much attention. I won’t even describe past seasons, since only the nucleus remains from 2010. But this season brings plenty of evidence that when the Celtics get backed against the ropes, they respond with a series of haymakers. Injuries or heart issues ended the seasons of Jermaine O’Neal, Chris Wilcox and Jeff Green. Doc Rivers simply plugged the next guy into the rotation and his team kept improving. Ray Allen went down, so Avery Bradley stepped in (and up). The Celtics were 15-17 at the All-Star break, but Doc Rivers swore — both to the media and to Danny Ainge behind closed doors — that his team would turn its season around. They finished with the league’s best record after the break.

This team was so confident it chose not to reach for home court advantage. The Celtics approached the playoffs with a chance host the Hawks in the first round, but they decided to rest regulars down the stretch. Their reasoning, besides having a number of nagging injuries: We’re good enough to beat the Atlanta Hawks — anywhere, any time. Now the Celtics still have home court advantage against Philadelphia. It’s safe to assume they still believe they’ll beat the Sixers two times in the next three, no matter how many times Evan Turner dashed into the paint Friday night.

Celtic Pride, at least in the Big Three era, is both a gift and a curse. It enables the Celtics to predict sun even when everybody else in the world sees a wild rain storm approaching. It also enabled the Celtics to collapse and look indifferent during the second half of a game that would have all but put the Sixers away.

The Celtics believe they’re going to win two of the next three games. Now, they only have to do it.