[2] Hadassi acquired his reputation through his treatise Eshkol ha-Kofer or Sefer ha-Peles, on which he began work on 9 October 1148.
Hadassi enumerates, under the head of each of the Ten Commandments, a complete series of coordinate laws; and the whole work is mapped out according to this plan.
The first commandment (alphabets 1-95) affirms the existence of God and covers the duties of the created toward the Creator, dealing, for instance, with prayer, repentance, future punishment and reward, and resurrection.
In fact, this part of the work is a compendium of religious philosophy, astronomy, physics, natural history, geography, and folk-lore.
Here Hadassi refutes the views of other sects; for example, the Christians, Rabbinites, Samaritans, and Sadducees, who maintain the eternity of the world.
Considering phonetics as necessary for the interpretation of the Law, Hadassi devotes to this study a long treatise, in the form of questions and answers.
Obviously his model was Nissim ben Noah's Bitan ha-Maskilim, or Peles Bi'ur ha-Mitzvot, written 370 years earlier.
The sources upon which he drew included the Ma'aseh Bereshit of R. Ishmael ben Elisha; the Baraita of R. Samuel of Nehardea, for astronomy; the Josippon for history; David ben Merwan al-Mukkamas' work on the sects; Eldad ha-Dani, for legends; while for grammar he utilized especially the Karaite grammarians, though he also made use of the Rabbanites, quoting Judah Hayyuj and ibn Janah.
P. F. Frankl, however, agreed with Firkovich in regarding it as a part of the "Eshkol ha-Kofer," which Hadassi had previously written in prose.