Basically, because all phenomenological problems are described in the form of differential equations, for example Newton's law of cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its environment. The equation is given by:
[tex]\begin{gathered} \frac{dT(t)}{dt}=-r(T-Tm) \\ \end{gathered}[/tex]A more basic example is velocity, which is defined as the change in position with respect to time:
[tex]v(t)=\frac{dx}{dt}[/tex]As we can see, all the physical phenomena that surround us can be described as a variable that changes depending on another.