In the Mistborn series, some people can heal themselves if they stored “health” in gold (usually in the form of jewelry) prior to getting injured. The way it works is you only heal back to whatever state your mental cognition agrees constitutes your body just prior to getting injured. So if you had lost an eye a decade ago and now wear an eyepatch, and accept that that not having a working eye is what makes you you, but then get stabbed in the gut and need to heal, you will only heal the stabbed gut, and not the eye.