This is false. I studied Maths at Cambridge and learned some courses completely on my own.
Anyway, this is almost irrelevant to what I was saying. Even if you assume every person can reason well, I'm saying you could still be in error unless you seek validation and guidance. It's really easy to think you've given a solid argument for something, but actually be wrong. It happens to everyone.
*By the way, there is a definite trend in physics for math, instead of experimentation, to be leading the way towards discovery.
As such, I think it's ridiculous to go up to anyone and tell him/her to just "study by yourself", I could give a maths book to a biologist and that person would understand absolutely nothing without guidance.