Because it really comes from this idea of a slope, which is change in y over change in x. As you'll see in future videos, one way to think about the slope of the tangent line is, well, let's calculate the slope of secant lines. Let's say between that point and that point. But then let's get even closer. Let's say that point and that point, and then let's get even closer, and that point and that point, and then let's get even closer. And let's see what happens as the change in x approaches zero. And so using these d's instead of deltas, this was Leibniz's way of saying, hey, what happens if my changes in, say, x become close to zero?