A representation of visual motion convenient for recognition shouldsmake prominent the qualitative differences among simple motions. Wesargue that the first stage in such a motion representation is to makesexplicit boundaries that we define as starts, stops, and forcesdiscontinuities. When one of these boundaries occurs in motion, humansobservers have the subjective impression that some fleeting,ssignificant event has occurred. We go farther and hypothesize that onesof the subjective motion boundaries is seen if and only if one of oursdefined boundaries occurs. We enumerate all possible motion boundariessand provide evidence that they are psychologically real.