That long -- very long -- weekend on Long Island finally wraps Monday, and CBS has said little of the end.

We do know this much: Robin (Cobie Smulders) finds the treasured locket; Barney (Neil Patrick Harris) is finally ready to tie the knot with her. Marshall (Jason Segel) and Lily (Alyson Hannigan) have their own chores on this momentous occasion. And Ted (Josh Radnor) wraps the shaggiest dog story you ever heard.

"How I Met Your Mother," ending nine pretty good seasons tonight, was the rarest of birds in the TV kingdom: a sitcom based on a philosophical principle.

This principle was the law of causation, or causality, which holds that a second event is caused by the first. It sounds trivial except that it's been a staple of Western philosophy going back to Aristotle.

And in the hands of creators Craig Thomas and Carter Bays, and longtime director Pamela Fryman, the adaptation of the law has been amusing, exhilarating -- and at the darnedest moments, unexpectedly moving.

The future Ted Mosby (voiced by Bob Saget) offered a reasonably concise overview of causation-as-applied-to-sitcom back in the fourth season when he told the kids, "The universe has a plan for you, and that plan is always in motion. A butterfly flaps its wings, and it rains. All the little parts of this machine are working, making sure you end up exactly where you're supposed to be, exactly when you're supposed to be there."

"Mother" has dazzled with its time manipulation, and often confused. But it was also implicit in the philosophy: that life will take you in strange, unexpected or funny directions.

As smart as "HIMYM" could be, this show never won Emmys, and the reason may be this: It could be silly, vulgar and inconsistent.

But it always was kind and gentle. This was one of those rare shows filled with a boundless sense of optimism and the redemptive power of love.

"Love doesn't make sense," Ted told Robin just last week. "You love Barney. He loves you. That doesn't have to make sense to make sense."