The trend line on Ben Carson's presidential campaign is going in the wrong direction. After finishing fourth in the Iowa caucuses on Feb. 1, he limped to an eighth-place finish in the New Hampshire primary eight days later. On Saturday in South Carolina, Carson finished sixth -- but only because there were just six candidates still running.

All of which makes Carson's speech following the Palmetto State primary all the more, um, well, interesting?

"There are news people here who think I'm going to make a concession speech," Carson said. "This is a just-the-beginning speech." But, wait! There's more. “People are being easily manipulated and told what they are supposed to think and who they are supposed to follow when in fact we have this tremendous brains with these enormous frontal lobes,” Carson added.

It's been obvious for several months that Carson isn't going to be the Republican nominee. (The national spotlight shone on Carson briefly last fall but moved on quickly when it became clear his foreign policy knowledge was close to nonexistent.) And yet, Carson is in Nevada now, campaigning in advance of Tuesday's Republican caucuses in the state.

Why?

Good question -- and one that lots and lots of people have asked me over the past few days. I have three possible explanations:

1. Carson's campaign has been living off the land for a while now. Yes, Carson has raised eye-popping amounts of money. He's also spent massive amounts raising it. Which leaves him with not much left.  His staff is rightly described as skeleton at this point. Costs are likely low. Expectations are even lower. Unlike, say, Jeb Bush, who would have been piling up massive debt while risking his family good name by staying in the race beyond South Carolina, Carson is traveling light in almost every regard. So, why not stay in?

2. Carson believes he was fundamentally wronged in Iowa by Ted Cruz and is looking for retribution or redemption or both. There's no question that Carson believes the whisper campaign from Cruzworld that he was dropping out of the race even as the Iowa caucuses were happening cost him votes. (He's probably right.) Carson may well be sticking around in the race in either a vain attempt to resurrect his good name or in a not-so-vain attempt to weaken Cruz as the race continues. While the latter option makes more sense, it feels decidedly un-Carson in that he has shown a lack of political calculation in this race. If he is driven to stay in the race by a mistaken belief that he can right the wrongs of Iowa and somehow wind up as the party's nominee, he is badly mistaken.

3. Carson really believes that he is on a higher mission that will bear fruit eventually. Carson's bid has always been more cause than campaign -- never more so than in the face of the increasingly depressing results of Iowa, New Hampshire and South Carolina. His Twitter feed is a constant reminder that Carson's belief in his own ability to turn the entire political system on its head remains very much intact.

Carson still thinks that his vision for the future of this country is the only way forward -- and that electing someone other than him could jeopardize the very foundations of America. If you believed that, you'd probably keep running for president, too.

Will Carson reconsider his "this is the only beginning" position if he finishes fifth (out of five) candidates in Nevada? Maybe -- especially since his longtime friend Armstrong Williams seems to be admitting the obvious even before the Silver State vote on Tuesday.

But I wouldn't bet on Carson leaving the race on Wednesday. After all, if he didn't get out after South Carolina, what will have changed in the proceeding three days? Carson has virtually nothing to lose by staying in the race -- do you think he really cares what the political class thinks of him? -- and still is under the impression that circumstances could change drastically. He's wrong about that, of course, but he's not going to be changing his mind anytime soon.