As the popular axiom goes, the definition of insanity is doing the same thing over and over again and expecting different results.

As present-day internet slang goes … well, the definition is different. Instead, “insane” is a catch-all for any number of adjectives. Ridiculous. Unbelievable. Exciting. Unfair. Incomparable.

How else would you describe the pairing of Real Madrid and Paris Saint-Germain in the Champions League Round of 16?

Real Madrid is amid arguably the greatest stretch in club soccer history, with three continental crowns in four seasons when the game has never been more competitive. PSG spent the summer splashing godless sums of cash on superstars like Neymar and Kylian Mbappe to chase such lofty heights.

This is a matchup we were looking forward to as a potential final or semifinal, an edict on both the state of Real Madrid’s run and the proximity of PSG’s pursuit.

And now we get them squaring off this early?

It was always a possibility, of course, because the Round of 16 pairs group winners with runners-up, and there are always big clubs that finish second in their groups for one reason or another. This year Real Madrid finished behind Tottenham in the loaded Group H.

But whether by fate, or fortune, or some conspiratorial ping-pong ball sleight of hand, the Round of 16 has been largely devoid of mega-clashes since its inception in 2003-04.

Big clubs have faced each other, sure, but rarely when both were on such spectacular runs of form. (And yes, we’re well aware Real Madrid is stumbling mightily in La Liga at the moment.) Barcelona and Chelsea squared off in 2005-06 when both won their domestic leagues and Barca won Europe, while a few years later in 2009-10, the league champion Blues faced treble-winning Inter Milan in the Round of 16.

The 2004-05 competition was a particular bloodbath of eliminating giants early, with marquee matchups like Barca-Chelsea, Manchester United-AC Milan, Juventus-Real Madrid and Bayern Munich-Arsenal.

But for sheer entertainment value and shock quotient, Real Madrid and PSG being paired together this season takes the cake.

The matchup might as well be a mirror image. While Real Madrid is the far more accomplished side, both historically and as presently constituted, PSG has an attacking trident of Neymar, Mbappe and Edinson Cavani to match Real Madrid’s robust contingent of Cristiano Ronaldo, Karim Benzema, Isco and others. Ronaldo is the reigning Ballon d’Or winner, and Neymar seems most likely to break up the monopoly on the award Ronaldo and Lionel Messi have established.

Both squads are replete with playmakers in midfield, and the defenses are sturdy enough and have been practically identical statistically. Factoring in per-game averages of shots allowed, tackles, interceptions, fouls and offside whistles, Real Madrid possesses a net rating of 7.05 and PSG isn’t far behind at 7.18.

Tactically, both teams are stacked enough to play multiple formations and press at different paces, whether coiling and springing lethal counters or overwhelming inferior opponents with skill and work rate.

So one can see why this two-leg affair is enticing. One can also ask if it’s good they’re meeting so early.

The answer depends largely on the individual. When the dust settles here, one of the five favorites to win the Champions League will be eliminated. That will be a great bit of drama in the moment. It will also deny fans the chance to see these two teams battle with a berth in the final or even the trophy on the line. That’s the double-edged sword at work in the knockout stages. Upsets and early mega-clashes mean unexpected twists, and they also stock the latter stages with uninspiring and unattractive teams.

But that’s a problem for a different day. Real Madrid vs. Paris Saint-Germain, pitted against one another in winter 2018, is as insane a Champions League Round of 16 matchup as there’s ever been, for a variety of reasons.

Affix whichever you like.

Joey Gulino is the editor of FC Yahoo and moonlights as a writer. Follow him on Twitter at @JGulinoYahoo.