mind of one's own, have a
Think independently, reach one's own opinions or conclusions. For example, You can't tell Karen what she should wear—she has a mind of her own. Also see know one's own mind.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.