Try it out. Take a mirror, put it very close to your eye but angled sideways, since you can obviously not look through your head.

You will have no issues at all focussing on what you are looking at, since you aren’t looking at the mirror at all.

You can also try that while looking at yourself through a dirty mirror. You can either focus on the dirt on the mirror or on your face. You can’t see both the dirt and your face in focus at the same time.