Most changes in the world pass us by without notice. Our sensory organs and perceptual apparatus filter them out, so they do not clutter our thoughts with irrelevancies. Other changes enter our minds as observations. We reason forward from them to deduce their consequences, and we react to them if necessary. Most of these observations are routine, and our reactions are spontaneous. Many of them do not even make it into our conscious thoughts.

But some observations are not routine: the loud bang in the middle of the night, the pool of blood on the kitchen floor, the blackbird feathers in the pie. They demand explanation. They could have been caused by unobserved events, which might have other, perhaps more serious consequences. The loud bang could be the firing of a gun. The pool of blood could have come from the victim of the shooting. The blackbird feathers in the pie could be an inept attempt to hide the evidence.