In the preceding section, we took the positive direction (for acceleration and velocity of raindrops) to be downward. In this section, where we are concentrating on a differential equation (rather than on physics), we take the positive direction in our figures to be upward. This is a purely arbitrary choice -- there is no inherent reason to prefer one direction over the other, except as a matter of convenience. Our independent and dependent variables are still `t` and `v`, respectively, but it really doesn't matter whether they represent time and velocity or not.