Unbelievable: With homelessness on the rise, Mayor Bill de Blasio is slashing spending for shelters in his 2017 budget — on the theory that his new policies are sure to turn things around.

As a matter of simple humanity, even a drop in homelessness wouldn’t justify major cuts here. Shelters today are too often horrible — so crowded and unsafe, you can understand why some would rather chance it on the streets, at least in the warm months.

And it’s sheer lunacy to rely on any new policy working — and working fast enough to justify cutting back.

Yet de Blasio aims to cut 27 percent from shelters for adults and 14 percent for family shelters. Fine, he pleads that “we’re still evolving” the budget — but why start “evolving” from nonsense?

The city’s Independent Budget Office, which blew the whistle on the mayor’s move, estimates the 2017 homeless population will be larger, not smaller — and shows that the performance of de Blasio’s new programs leaves zero hope they’ll make the difference he’s assuming.

Whenever demand for shelters surprises City Hall, it has to scramble — spending big to put up homeless in hotels, and OK’ing improvised shelters with even less regard for quality and safety than in existing ones.

De Blasio spent much of last year in denial about the surging homeless problem, even mocking The Post’s extensive coverage, before finally admitting the truth.

He plainly didn’t really learn his lesson — he’s moved from acceptance to magical thinking.