Cubs starting pitcher Jake Arrieta has a 0.84 ERA this season. (Gene J. Puskar/AP)

Jake Arrieta’s baseball life is a series of events that are cast against those from last summer, the absurd jumble of numbers that established him as one of baseball’s best pitchers. Sample: When Arrieta walked four batters in a start last week, it was tempting to point out that he had walked four batters in the entirety of September last year.

His starts are events. His development is a curiosity. His future, after the National League Cy Young Award in 2015 and a 1.00 ERA through his first five starts this season? Why put limits on it?

“This is going to be hard to fathom,” veteran catcher David Ross said. “He’s not even really locked in yet with his command.”

Here we are, then, with Arrieta, with the possibility to be dazzled every fifth day. Tuesday night, he took the mound at PNC Park for the first time since he threw a five-hit shutout against the Pittsburgh Pirates in the National League wild-card game last October.

He promptly missed the strike zone with eight of his first 10 pitches, walking the first two Pirates.

So, then, reach into the bag and pull out a corresponding Arrieta oddity, the little trinkets that invariably paint him with a favorable shade. Here’s one: In 19 of Arrieta’s 33 starts last season, he walked fewer than two batters.

By night’s end, those two walks were another footnote. He had seven scoreless innings in which he allowed just two singles. His ERA dropped to 0.84, the best in baseball. He has won all six of his starts, including 7-1 on Tuesday.

“Whether he’s at that level that we saw those last couple months of last year, or pitching more like he did the first half of that season, it’s just different flavors of excellence,” said Theo Epstein, the Cubs’ president of baseball operations. “He’s established himself as an excellent pitcher now. That’s who he is.”

Sorry to do this (again), Orioles fans, but isn’t that an amazing paragraph to read? In 3½ seasons at Camden Yards, Arrieta made 63 starts and pitched six more times in relief. His ERA: 5.46, before he was traded in 2013 (for Steve Clevenger and Steve Feldman, no less).

With the Cubs, in 73 starts, it is 2.14. His WHIP (walks and hits per inning pitched) with the Orioles: 1.472. His WHIP with the Cubs: 0.922.

“Before, in Baltimore, we saw him as a guy that you just need to wait him out,” said Cubs utility man Ben Zobrist, who faced Arrieta regularly when he played for Tampa Bay. “You’re probably not going to get a lot of hits off of him, but if you wait, and you get into a 3-1 or 2-0 count, you might get a good pitch to hit and you might be able to drive one. We would score based on him walking people.”

Lest we think this is revisionist history, Zobrist has 21 career plate appearances against Arrieta. He walked in nine of them.

Told Pirates Manager Clint Hurdle had inserted lefty-swinging Matt Joyce into the lineup because Joyce had gone 7 for 14 against Arrieta back when he played for Tampa Bay, Arrieta summed up his career arc perfectly: “He’s facing a different guy now.”

So different that those first two walks were hardly a concern. Why did they happen? Arrieta realized early that his sinker had too much movement on it. This, though, is what Ross was speaking about: In these first half-dozen starts of the season, Arrieta is still working through his delivery — and getting results anyway.

“He’s a guy that likes to be out there every fifth day without any extra days so he can make sure he has his release point, the right angle, all of it locked in,” Ross said.

What might he look like then? Well, like the guy who went 11-0 with a 0.41 ERA , allowing four earned runs over his final 12 starts of 2015. But think about the idea that Arrieta is still coming into form this season.

His response to Tuesday’s walks: Retiring 19 of the next 20 hitters he faced, including just three balls that left the infield. When Pirates catcher Francisco Cervelli finally ended the clinic with a two-out single in the seventh, the Pittsburgh fans sarcastically cheered.

“It’s rare that you’re around somebody that’s pitched as well as he has for that extended period of time,” Cubs Manager Joe Maddon said. “. . . Honestly, he’s going to continue along this path.”

On his 99th and final pitch of the night, Arrieta threw Gregory Polanco a 2-2, 93-mph sinker that caught the lower part of the zone.

It is part of what makes catching Arrieta so much fun, Ross said, because he can throw any of his pitches — four-seam fastball, two-seam sinker, slider/cutter, curveball and an occasional change-up — in any count, and they’re all superb.

“With someone else, you might be like, ‘Man, I’d really like to call a change-up here, but I don’t know if he’s going to throw me a good one,’ ” Ross said. “With Jake, you don’t have to ask. Any of the above. Just throw it.”

So, then, where to find concerns? Late in that wild-card game last year, the Pirates stung some balls off Arrieta.

That start gave him 238 innings on the season. In his two remaining postseason starts, he allowed eight runs in 10⅔ innings.

“What we saw last year is that he’s human, that pushing 250 innings he can tire,” Epstein said. “We don’t want that to happen again ever, especially at the most important times of the year.”

For now, though, it’s early May. The Cubs have baseball’s best record. They may, too, have baseball’s best pitcher. On Sunday, he’ll start again against the Washington Nationals — another event in a summer full of them.

barry.svrluga@washpost.com