Training when injured...

It's so hard to know if it's okay to train when hurt. Our bodies tell us one thing while our minds tell us the truth. Or is it the other way. I am currently injuired with 2 bad knees. I will need surgery on one of them. My body told me not to train a few weeks ago . It was just impossible and unsafe. As time passed by, my mind is telling me that I can train but within reason.