Orthodox Judaism holds that the words of the Torah, including both the Written Law (Pentateuch) and those parts of the Oral Law which are "halacha leMoshe m'Sinai", were dictated by God to Moses essentially as they exist today. The laws contained in the Written Torah were given along with detailed explanations as how to apply and interpret them, the Oral Law. Although Orthodox Jews believe that many elements of current religious law were decreed or added as "fences" around the law by the rabbis, all Orthodox Jews believe that there is an underlying core of Sinaitic law and that this core of the religious laws Orthodox Jews know today is thus directly derived from Sinai and directly reflects the Divine will. As such, Orthodox Jews believe that one must be extremely careful in interpreting Jewish law. Orthodox Judaism holds that, given Jewish law's Divine origin, no underlying principle may be compromised in accounting for changing political, social or economic conditions; in this sense, "creativity" and development in Jewish law is limited.
What is the law that is considered the core of religious laws Orthodox Jews know today?
Sinaitic law