To Have a Mind of One's Own

This idiom has my right knee written all over it. You'll see why.

Meaning
It means to be capable of making decisions without advice. [Yep ... that's my knee.]

Use in Normal Speech
When discussing akward people/body parts.

"My right knee has a mind of its own. It started to hurt yesterday evening and I don't know why."

When expecting someone to make a decision without help.

"I thought you had a mind of your own, mate. Use it."