Most people describing the time dimension rely on an analogy with 3-dimensional space, i.e. time is like walking across a field, whereby we humans are generally limited to traveling in the positive direction.

Continuing the analogy to 3-dimensional space, if we were allowed to travel backwards across this field of time again, what prevents us from effecting changes in this “earlier” section of the field?

The very act of going back in time involves displacing air molecules presumably, which counts as changing history, albeit in an insignificant way. Since there exists such obvious exceptions to the stated rule, it seems presumptuous to stand by that generalized statement.

It puts me in mind of the quantum mechanical idea that something only obtains a fixed state upon observation. In that sense, history has all been observed before, and must therefore be fixed and immutable. But by that logic, time travel becomes either impossible or pointless.

I generally prefer Douglas Adams’ explanation of it by the character Slartibartfast. If you start meddling with time, it’s just all going to get severely muddled up and confusing. :)