know one's own mind - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Be certain about what one wants; be decisive. For example, Don't ask him; he's so tired that he doesn't know his own mind, or She certainly knows her own mind when it comes to giving stage directions. This term was first recorded in 1824.