Jim Carrey -- Dumb and Dumber To ... THIS IS REALLY HAPPENING!!!

Yes, we're saying there's a chance -- in fact, more than a chance ... Jim Carrey just gave the world hardcore proof the "Dumb and Dumber" sequel is really, finally filming.

Jim posted a WhoSay pic of himself and Jeff Daniels -- aka Lloyd Christmas and Harry Dunne -- attempting to read Jim's new children's book called, "How Roland Rolls."

It looks like they're standing in between their trailers on the movie set ... which is a good sign cameras are already rolling on the long-anticipated (almost 20 years now) "Dumb and Dumber To."