Rabbi Yaakov ben Asher, born in Cologne, Germany, around 1269 and died in Toledo, Spain, in 1343, wrote, among other books, the Arba Turim, a compendium of all the practical laws today (he left out the laws that depend on the Temple, or having a King, etc.) arranged in four "columns," as he called them.
These four sections are subdivided into chapters and each chapter into subsections, thus covering all of current Jewish law.
Rabbi Yosef Caro (Toledo 1488, Tzfat 1575), author of the Code of Jewish Law, known as the Shulchan Aruch, used this same order to codify the main text of today's practical laws, studied by all Jews worldwide.
Now, they say there is a fifth part that was never written, but is as important as the other four, and that part is common sense.
To illustrate this with a personal story: once, after Yom Kippur, we were invited to dinner at a friend's house. After a lavish meal, my wife threw up everything she had eaten. Now, if one has eaten bread and is full, it is a Torah commandment to recite a blessing (Birkat HaMazon), but, look, if one has thrown up everything, one is not truly full—quite the opposite!—but one ate and was full!
We presented our question to a rabbi: Should my wife recite the blessing after meals in this case, or not? The rabbi replied: "Take your wife home, make her some tea and toast, and when she feels better, have her say the blessing.".
The answer is based primarily on common sense. The person is feeling unwell; the first step is to resolve that situation, then resolve the halakhic dilemma.