The most basic hermeneutical methodology is the kal v’chomer, known in Latin as an argument a fortiori, meaning “from the stronger case.” The nature of an argument a fortiori is “Arnold is known to be stronger than Barack. If Barack can lift 100 lbs., a fortiori Arnold can lift 100 lbs.” There is a limit to the argument a fortiori, however: you can’t get more out of it than you put into it. So, I may know that Arnold is stronger than Barack, but I don’t know if that means Arnold can lift 200 lbs., 150 lbs., 101 lbs. or 100 lbs. and 1 oz. All I know for sure is that Arnold can certainly lift whatever Barack can lift.
Now, let’s see how a kal v’chomer works in Biblical exegesis.
The Torah tells us that Yom Tov is just like Shabbos except that food preparation is permitted. In other words, the laws of Shabbos are known to be more stringent than those of Yom Tov. If something is known to be permitted on Shabbos, kal v’chomer it must be permitted on Yom Tov. Conversely, if something is known to be prohibited on Yom Tov, kal v’chomer it must be prohibited on Shabbos.
The Torah itself gives us a perfect example of a kal v’chomer. In Numbers 22:14, after reprimanding Miriam, God sent her out of the camp for a week. Moshe asked for clemency for his sister but God replied, “If her father had spit in her face, wouldn’t she hide her face in shame for seven days?” If she would hide for a week after being chastised by her father, kal v’chomer she should leave the camp for a week after being chastised by God. (Logically, she should have had to go out for two weeks or more but remember, you can’t get more out of a kal v’chomer than you put into it!)
Since a kal v’chomer is a strictly logical process, anyone can argue a case a fortiori.