BOSTON – The NBA regular season proved that the Sixers have come a long way in the five seasons of The Process, but the playoff portion of the schedule pointed out just how far they still have to go.

Playing against a team that lost its leading scorer near the end of the season, the Sixers were unable to get past the Boston Celtics in the Eastern Conference semifinals, even though statistically the teams were fairly even and three of their four losses were decided by five points or less. Winning those games rather than losing them is a studied art, and the Sixers are still in school on that one.

"The thing I remember telling them when the playoffs started was, 'You will learn more about yourselves in the next few weeks than you ever learned in your career,' " coach Brett Brown said. "It's expedited learning in a high-pressure environment."

Wednesday's 114-112 closeout for the Celtics was the tightest and most pressurized of them all. The Sixers came back from 12 points down in the third quarter, but they couldn't finish off a win that would have extended the series to a sixth game in Philadelphia.

The game seesawed through the fourth quarter, with Boston appearing to take control, and then the Sixers coming back with big shots to hold as much as a four-point lead with two minutes to play. They couldn't keep it, however, and two late turnovers allowed Boston enough chances to get the win in front of a roaring TD Garden crowd.

"We didn't go away," Brown said. "I get what we've done this season, but there's an emptiness you feel when somebody stops you from doing what you like doing with guys you like working with."

The series was an experience that the Sixers needed for the future. What works during the regular season isn't necessarily reliable in the postseason. The Sixers lived and usually thrived with their three-point shooting, as they racked up 52 regular-season wins. But the defense gets better in the playoffs, and the best teams respond. Boston's overall shooting was better that it was during the regular season. The Sixers' was worse.

If a three-point shooting contest was expected to finally materialize on Wednesday, it never did. The Sixers had been outscoring Boston in the paint all series long, and, despite Brown's frequent proclamation that "the three is king," his team looked content to bang the ball inside, hope to get to the line, and win the rebounding wars.

Unfortunately, those tactics tend to work better at home than on the road. The shooting, both from inside and the perimeter, was pretty even on Wednesday night, but the Celtics were not harassed into turnovers as they had been the previous two games in Philadelphia. Boston took slightly more shots from the field because it took better care of the ball and had a significant advantage at the foul line as well.

That was enough to narrowly decide the game. Brown was right  — his team didn't go away. The players scrapped and tried, but ultimately they were unable to overcome their collective inexperience. The effort was admirable, but it wasn't enough, not for this game and not for the entire series. Good, but not good enough for the playoffs.

It might be a bit early for too much perspective, because the elimination is so fresh and the end of the first postseason run of The Process Era came to such a sudden end. But this is a team that had a 15-19 record in late December, and its ability to even qualify for the playoffs was doubtful.

To travel from there to the conference semifinals in just over four months was remarkable and indicated how much more went right than wrong in what was always expected to be a pivotal season in the team's development.

Joel Embiid stayed relatively healthy. Ben Simmons operated well as the primary ballhandler. The role players arrayed around those two performed pretty much as expected, and Brown was able to coach a style that became dependable and effective most nights. The Sixers pushed the pace, spread the floor with shooters, and played defense. Embiid was the X-factor, but, after all, that is what he was drafted to be.

The end is a disappointment, but the season certainly wasn't.

"You think about the journey they've been on," Brown said. "Initially, you want to make the playoffs, and then you want to finish fourth [in the conference]. Then, you get greedy and want to finish third. Then you win 50 games, then 52. Then you just want to win a game against Miami, two games, and we come out of it 4-1. We win 17 in a row to end the year. We took off, bam, and with that, expectations went with us. It's human nature."

To be honest, the expectations didn't seem outlandish, given the win streak that ended the season and the way the team played against the Heat in the first round of the playoffs. Boston was a higher seed, sure, but most of that was earned with Kyrie Irving on the court and not sidelined with a bad knee.

It turned out that the matchup wasn't good for the Sixers, though. The Celtics were faster and more athletic, with their bigger players more able to step out and prevent long-range shots. It also turned out that the learning hasn't ended for the Sixers. In fact, it has just begun.