This seems to be plagued with ISFJ stereo typing that is mostly not true. I could see an ISTJ maybe falling under these definitions, however this is not the greatest separation of ISFJ's and INFJ's.Let's see...
INFJ's find truths by thinking about things. We may space out for ten minutes to think over something, and draw a very insightful conclusion from our thoughts that we apply in our life going forward.
We get upset when there is a law that is unfair or unjust. We want everyone treated fairly and similarly. When things don't work this way, we'll get far more upset than any other personality type.
See an ISFJ is likely to take a law at face value. This is the law, it has been around for a 100 years, everyone knows about it, it's set in stone. Considering that it should be changed may never cross their mind.
An INFJ would analyze the law and how it affected society, they would then compare this law against an ideal of how things should be. If it doesn't meet the ideal, then it's a bad law and we want it changed. Just as an example, I'm utterly disenfranchised with the way the US government operates because of the pervasive corruption that now exists. Other people see this as the way things have always been, but I don't care how it was, I look at it today and it's corrupt, and getting worse every year. Rules that were long-held to be essential to the operation of the congress are being ignored because parties in charge bend the rules to favor themselves. Corporations now get to fund political campaigns....
You see how much this bothers me? I almost turned the entire post into this.
See an ISFJ is less likely to concern themselves with fairness as far as the law is concerned. Their dominant Si generally leads them to accept what is; they're not constantly imagining how things could be improved. If I point out to my ISFJ friend, why the government is corrupt and unfair, he likely agrees with me and then digs up a few details that support my claim. Suggesting how to change it, and how it should operate is a bit out of his league. He could do it if he really thought about it, but he isn't going to trouble himself with that. As an INFJ, I can't help but trouble myself with it.
Of course INFJ's pick their battles when it comes to fairness and justice. We all get worked up over different topics, but that gives you an idea of what having a strong Ni does when paired with Fe and Ti.
ISFJ's are often thinking as well, and we often see the value in laws, but if we see a law as harmful, then we are not going to just step in line, just because its been around for a period of time. We are not mindless automatons. Si is a judging function. We are constantly taking in the details of things and comparing them with past experiences. We definitely care about what is fair and what is not, but what we see as fair might not coincide with what you see as fair. The underling principles are often important to us.
ISFJ's are just as likely to get fired up over a subject that resonates with them, yet they also pick and chose their battles because they we also value harmony. Just because an ISFJ does not disagree with you on the surface, does not mean they agree with you. They just want to spare your feelings and avoid unnecessary conflict. Likewise, if an ISFJ is quite capable of understanding the nuances of how a government should run, but it does not mean they are going to force their views of how things should run on others.
You can generally tell the difference between an ISFJ and an INFJ buy their speech and their mannerisms. Its hard not having a person face to face to type them properly. The best thing, I would suggest is to type a essay, of a descent length. Have others go over it and see what conclusions they draw and why.