Question:"for even during Chol Hamoed, when the prohibition against melachah is of Biblical origin, the Rabbis permitted in a case of loss." The notes in the Artscroll Gemara (11b2, note 12)implies the following: Rashi holds that since it is Biblical, we begin with prohibitions and the Rabbis allow. However, Tosafos and Meiri imply that the prohibitions against melachah are Rabbinic and are supported by an asmachtah. Hence, we begin by allowing all and the Rabbis prohibit. When a ruling is made today, the rule maker must start from only one of the viewpoints. Which viewpoint does he begin with? Is this different (as is often the case) with the viewpoint at the time of the Gemara?
In regard to Chol Hamoed we follow thew the strict viewpoint -i.e. that the issur is biblical-since many Rishonim hold this view. [The Mishna Brura beginning of Hilchos Chol Hamoed.]
In regards to contemporary rulings our approach is always to assume something is permitted until it can be proven forbidden [either from a verse in the Torah or from the Gemara.] There are a few areas where our Rabbis were strict because of precautions that they deemed necessary. In doubtful situations, where we are not sure of the approach of chazal, On a Torah level we are strict and on a Rabbinical level we are lenient.