An older golfer has marked his ball on the green, but not replaced it. He studies the line and then stands over the mark as if ready to putt.
He takes a stroke over his marker and then goes “Oh, I guess I need a ball.” Since he intended to hit a ball that wasn’t there, should he have counted a stroke?

Les' Answer:

By definition, if it is his intent to hit the ball it would be a stroke. However, because of the strict literal interpretation of the English language that is used in the Rules, one must have "the" ball to make a stroke at "the" ball. Since "the" ball is not in play, this could not be a stroke.

Stroke
A "stroke" is the forward movement of the club made with the intention of striking at and moving the ball, but if a player checks his downswing voluntarily before the clubhead reaches the ball he has not made a stroke.