puts on scholarly hat
Actually, there is one key difference between the ISFJ and the INFJ. How do you perceive the world around you?
Do you continually remember facts from the past, listen to your body's internal signals and senses, and take comfort in repeating traditions or tasks? Then you are an ISFJ
Do you continually seek insights into concepts to the point that you can easily see how a decision would affect yourself and others in the future? Do seek hidden truths behind symbols? Then you are an INFJ.
(Also, INFJ have to make it a point to make sure to take care of their bodies, while ISFJ are very highly attuned to their bodies, to the point they're very sensitive)