As one source acts on what is outside of it, multiple sources act on one another. They do so when their fields meet. For example, throw two pebbles into a pond and watch the interfering ripples: first, point sources encounter each other’s activities at the boundary of their expanding radiations; next, these pulses, when striking each other, are modified when struck. External modifications of a radiation affect its internal makeup. Since force is an active pulse, and since activity, for Kant, describes force better than anything else, a collision with another field has constitutive effects on the original activity.  (Consider weather—when air masses collide, they affect each other’s dew point, temperature, or pressure.) Hence Kant concludes that the action of force-points amounts to mutual changes of their internal states (#4; 1:19.4–6).