Even if you have an equation, doesn’t mean you can solve it. N-body problem is a perfect example.

You cannot determine all the Parameters of the equations accurately. Changing the parameters could drastically alter the output behavior. Finally, you don’t know the initial conditions too.

These are the challenges that classical physics have to deal with. Quantum is not even factored into, yet.

Welcome to the land of Nonlinear Dynamics. We have bifurcations, strange attractors and of course, whole lotta chaos.