For the past four years, the Boston Celtics have existed more in theory than reality. Whatever they accomplished was merely a prelude to a time when draft picks and free agents became tangible roster elements instead of dreamy fantasies. Hiding behind the space reserved in the rafters for Banner 18 is an existential quandary that has followed this team around all season: Does any of this really mean anything?

Here’s a test. If you were to ask Celtics fans back in October if they would be happy with a 53-win season that ended in the conference finals they would say, “Yes. Obviously.” They would want details, of course, and most of the details have been positive.

Isaiah Thomas became an All-NBA player and top-10 finisher in the Most Valuable Player voting. Prized free agent big man Al Horford showed his worth in the postseason after a solid, if occasionally uneven, first season in Boston. Avery Bradley continued to improve as an offensive player and was a defensive monster in the playoffs. Jae Crowder posted career best marks in three-point shooting, rebounding, and assists.

The young players also made contributions. Marcus Smart did so many Smartian things that he became an impactful player in the league even without a reliable jump shot. Second-year man Terry Rozier showed remarkable flashes of speed and power, becoming one of the best rebounding guards in the league, albeit in a limited role. Rookie Jaylen Brown offered glimpses of a ceiling many thought was beyond him when he was drafted, and he pushed through the rookie wall to earn significant playoff minutes.

There were things the Celtics did well and things that needed improvement, such as their wandering defensive intensity during stretches of the season. There were also fatal flaws, namely an inability to control the defensive boards and generate consistent offense without Thomas.

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Still, there was much to appreciate about the Celtics’ season and that’s before we get to the first overall pick they won in the lottery via the legendary Brooklyn trade.

Taking all those factors together, there is no logical way to argue that this has been anything but a smashing success.

And yet, the C’s were overwhelmed in the conference finals by LeBron James and the Cavaliers in five games that included three losses on their home floor by an astonishing total of 90 points. They tested the limits of their abilities, and it turned out to be exactly where everyone thought it was.

“We had a great year,” Brad Stevens said. “In some ways, we made a run at it. We made progress, but not good enough. And you know, I've said this before, if you coach in Boston, good enough is what matters.”

If LeBron and the Cavs are the measuring stick, then the Celtics failed their final test, just as Atlanta, Toronto, and so many others have before them. There is surely a case to be made that the only thing that matters is winning championships and everything else (even the super-fun stuff) is just for show. But that’s only half right.

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“I don't have any objectives other than winning the whole thing,” Stevens said. “To me, that's the only goal you shoot for because then if you don't, if you put your goals lower, then you create a ceiling for your team, and I don't think that's fair to your team.”

Fair, but there is space between the good and bad when judging a season in full. This is the territory that Stevens works as a coach, and this is where we really should examine their season.

After they beat the Wizards in seven games, I tried to get Stevens to bite on a big-picture question. He wasn’t having it. With Stevens, there are no mystical forces at work and the Basketball Gods find no favor here.

When I joked with him later that my goal in the playoffs is to get him to be introspective, Stevens casually replied that he has certain principles and he lives his life by those credos. The critical validation that comes with winning a couple of playoff series isn’t important to him.

Stevens’ principles involve simple things like honesty and clarity. They also work on deeper levels, such as his belief in the ability to evolve and improve every day through practice and experience. That’s where the interesting stuff takes place. Get at that, and you can understand what makes Stevens a good coach and what truly characterizes this team.

That Stevens is good at what he does is no great revelation. It’s obvious to even casual fans that he draws up super-neato plays out of timeouts that lend themselves to Zapruder-like breakdowns the next day. (The weak-side screen is back and to the left.)

Yet, the X’s and O’s are merely the textures on the canvas that make the painting come alive. The broader outlines of the portrait take a bit longer to come into focus but are no less illuminating. This is the light they need to be viewed in at the moment.

In full view, the Celtics displayed an admirable feistiness that played well in Boston (as it would everywhere), and that quality defined them across the league. Coaches worried about getting run out of the Garden, and opponents respected them because they’re a pain to play against. Whether they feared them is another matter.

Even the Celtics’ most ardent detractors gave them nods of grudging respect while hoping they get put back in their place by better teams. Including the postseason, they were 2-7 against the Cavs and 19-16 against the rest of the East playoff field, which was hardly the stuff of dominance.

They did, however, beat every team in the league at least once except San Antonio, Oklahoma City, and (oddly) Denver. So while they were sometimes overmatched, they were usually competitive. You can’t win 53 games by accident.

That secured the top seed in the East, and it was a weird achievement. They had their chance to take control of the race late in the season and were blown out by the very Cavaliers who gifted them the top spot with their lethargic disinterest down the stretch. That left the Celtics in the awkward position of defending their regular season status while needing to prove themselves worthy in the postseason.

What happened during the playoffs was validation, to a point. The Chicago series was harder than it should have been, and the C’s were fortunate that Rajon Rondo was injured during Game 2. When the Bulls took the fight to them, however, they responded. Bradley and Smart stood toe-to-toe with Jimmy Butler and Dwyane Wade, and they ripped off four straight wins.

The Celtics then went seven frantic games with the Wizards, which seemed right. The C’s rallied to win games at home, while the Wiz won convincingly on their floor. There was a draining closeout attempt in Game 6 that ended in a crushing defeat, but they persevered in a memorable Game 7 performance.

It was a great series, arguably the best of the entire postseason, and either team could have won. The Wizards have been proclaiming themselves superior ever since, but that’s a hollow boast. Expectations may be graded on a curve, but wins and losses are not.

Again there were positive individual signs. Horford came through like the max free agent he is with brilliant all-around performances. Even Kelly Olynyk — long the most beleaguered member of the team by the home fans — had his turn as a cult favorite and Game 7 hero.

Equally important was the play of Isaiah Thomas. Many people went into that series believing that John Wall was the best player on the court, and while he often was, Thomas matched his best moments and even surpassed them.

The debate over whether Thomas is a great player or just a small guy doing amazing things will rage forever and it may ultimately define their future course. During the 2016-17 season, however, Thomas was a marvel to watch and one of the single best things about the NBA experience.

His postseason run has to be viewed as the culmination of that incredible star trip. That he persevered through personal tragedy and injury to deliver heroic performances when many wondered how effective he’d be during the postseason is now the stuff of legend.

Getting past the Bulls and Wizards were notable achievements, and that’s where things stood heading into the conference finals. Even with home-court advantage, no one seriously gave them a chance to beat Cleveland and they didn’t come close.

They were blown out and embarrassed at home in the first two games, losing Thomas to a hip injury in the process. Coming back to win Game 3 in Cleveland was astonishing and an immense credit to their character. They had a shot in Game 4 but had no answer for the individual brilliance of LeBron and Kyrie Irving.

Back home at the Garden for Game 5, their closeout game had all the intensity of a regular-season blowout in December. That left a bitter taste to an otherwise fine season, and all of that brings us right back to the beginning.

They will try to get a star in the draft and maybe even one during the summer. There will be roster decisions that will bring clarity to their ultimate direction. The Celtics will keep evolving because they have to if they want to truly be among the elite. This season needs to be viewed in that context — as a bridge to another destination.

This was the season when the Celtics went from a hypothetical entity to team of significance, and it must also be said that the journey was a helluva lot of fun. They reached their potential, and even if it left them wanting more, it’s hard to ask for much more than that from an NBA season.