Having become renowned for espousing the cause of midrash ha-Torah, Hillel now established rules to govern this exercise, otherwise there would have been a danger that people would 'interpret' the biblical text out of all recognition. The idea was not to 'make the text say what I want it to say' (as is the case with so much modern exposition) but to ascertain and make explicit everything that might legitimately be considered to already be implicit in the text. He established seven rules (middot). These rules were later expanded to thirteen and even thirty-two, but Hillel was responsible only for the initial formulation of seven rules. Here we get a little technical, and I must ask you to be patient while I explain some of these rules, because they are part of the legacy that Hillel has left us, and part of his indelible imprint on the development of traditional Judaism.
The first of the seven is Kal va-ĥomer - logical inference from minor to major. We gave al description and illustration of this method in Avot 052, explanation #9.
We also illustrated the second method of the seven in the same shiur, although we omitted to give it its name. This method is called Gezerah Shavvah and a description and illustration is given in Avot 052, explanations 7 & 8. Further discussion concerning this rather problematic method can be found in Sanhedrin 009, Sanhedrin 054, and many other places.
The third and fourth methods are very similar. The third is called Binyan Av mi-Katuv Eĥad. Essentially, this means that when several biblical passages are thematically related but only one of them contains a very specific stipulation, we may assume that the stipulation applies to all the texts which are thematically related. We may illustrate this method by quoting Deuteronomy 19:15 -