Working in Shabbat - Building a 'fence' round the Torah

1. On page 248 of A Judaism for the Twenty-First Century (6th Edition), discussion on activities that are prohibited on Shabbat were so interesting how some were devised from 'what if' (ie a guitar may no be played, because what if a string breaks and it needs to be repaired...).  Would you say this 'what if' mindset is a characteristic of a Jew?  
2. Rabbi Jonathan responds: Yes, it is called 'building a fence round the Torah' - so the tradition says chicken is not meat (as it doesn't give milk), but what if you THOUGHT you were eating chicken but it turned out to be veal?! So the Rabbis banned chicken, but did it as a rabbinic ban, not a Torah ban, as they knew it was only a safeguard.