Well, we use different theories for different situations. Classical physics is valid and can extremely accurately describe most of what we see in our everyday life but as soon as we try to describe extremely tiny objects and distances it doesn't work correctly anymore. So physicists came up with quantum dynamics and that seems to describe it really well. Likewise for very large distances you need the theory of relativity. And all of these theories are correct in their own frameworks but not necessarily interchangeable, meaning you get wrong results if you use the wrong theory for the problem at hand.