 In this context, the complex notion of “determination” should be discussed, since it approximately corresponds to the composite direction of a body’s quantity of motion. In some passages, Descartes apparently refers to the direction of a body’s motion as its determination: “there is a difference between motion considered in itself, and its determination in some direction; this difference makes it possible for the determination to be changed while the quantity of motion remains intact” (Pr II 41). Yet, a single motion does not have just one determination, as is clear in his critique of Hobbes’ interpretation of determinations: “What he [Hobbes] goes on to say, namely that a ‘motion has only one determination,’ is just like my saying that an extended thing has only a single shape. Yet this does not prevent the shape being divided into several components, just as can be done with the determination of motion” (April 21, 1641; AT III 356). In the same way that a particular shape can be partitioned into diverse component figures, so a particular determination can be decomposed into various constituent directions. In his Optics, published in 1637, Descartes’ derivation of his law of refraction seemingly endorses this interpretation of determinations. If a ball is propelled downwards from left to right at a 45 degree angle, and then pierces a thin linen sheet, it will continue to move to the right after piercing the sheet but now at an angle nearly parallel with the horizon. Descartes reasons that this modification of direction (from the 45 degree angle to a smaller angle) is the net result of a reduction in the ball’s downward determination through collision with the sheet, “while the one [determination] which was making the ball tend to the right must always remain the same as it was, because the sheet offers no opposition at all to the determination in this direction” (see Figure 1).