Psychiatry and psychology are branches of human psychology -- both rely on the study of human psychology for validation. And if human psychology were a rigorous, empirical science, people wouldn't be able to say, as you just have, the psychiatry and psychology do not have much in common.

Would you say the same thing about cosmology and particle physics? They're very different -- one studies events at the smallest possible scale, the other at the largest possible scale. But no one suggests that they're unrelated to either each other or to their parent field of physics. The reason is they're sciences.

> Psychiatrists are medicals doctors treating the organ "brain".

No, that's false -- you just described the field of neuroscience (except that neuroscience is more a research than a medical field at the moment). Psychiatrists are psychologists with a medical degree, they are not neuroscientists, and they treat the mind, not the brain.

> It's pretty much interchangeable with neuroscience.

Not remotely. Neuroscience studies the brain and nervous system, psychiatry is a branch of psychology that studies the mind.

Source: http://en.wikipedia.org/wiki/Psychiatry

Quote: "Psychiatry is the medical specialty devoted to the study, diagnosis, treatment, and prevention of mental disorders."

Note the phrase "mental disorders," i.e. disorders of the mind. Not "brain disorders".