Wintry scenery is the starting point here, but the real subject is how unreliable our eyes are. The speaker describes snow falling in flakes that look bigger than the moon, which can't be seen. Then comes a strange image: four trees climbing a blue hill, repeated in twenty-seven rectangles, as if the view has been split into panes or frames.
Midway through, the tone shifts to something close to a lecture. The speaker says the brain can adjust to nearly anything, from reversed eyeglasses to loss, and that we never perceive the world as it really is. The evidence is simple: the moon is never that small, and hills are never that blue.
The final lines turn this into a kind of logic. If four trees can appear in twenty-seven versions, perception must be unstable. The mood is calm, curious and a little unsettling. The mention of "loss" hints that our flexible minds may protect us, though the poem leaves that thought quietly open.