One of the first things we learn in “Believing Is Seeing” is that its author, the filmmaker Errol Morris, has limited sight in one eye and lacks normal stereoscopic vision — “My fault,” he writes, for refusing to wear an eye patch after being treated for strabismus in childhood. It’s hard to think of another writer who so neatly embodies the theme of his own book. “Believing Is Seeing” is about the limitations of vision, and about the inevitable idiosyncrasies and distortions involved in the act of looking — in particular, looking at photographs.

Or anyway, it’s sort of about that. Reading it, I thought of Morris’s first film, “Gates of Heaven,” which is ostensibly about pet cemeteries and includes more material on pet cemeteries than any other movie ever made (including “Pet Sematary”) but is, nonetheless, not really about pet cemeteries at all. Likewise, “Believing Is Seeing,” though perceptive about photography, is fundamentally concerned with something very different: epistemology. Morris is chiefly interested in the nature of knowledge, in figuring out where the truth — in both senses — lies.