Hold your right pointer finger up to your face, about six inches in front your nose. First focus on your finger by crossing your eyes; then focus on the background. Move slowly from one image to the other. Notice how this focusing involves two different things: one is the merging or splitting of the double image, and the other is the blurring or sharpening of the edges.

Now try this: focus on the background, so that you see two blurry fingers, and then slowly bring the fingers into sharp focus but do not bring them together. See, if you can, a single blurry background, and a double sharp focused finger.