 Originally written in Arabic, this work is extant in two Latin (Liber de Definicionibus/Definitionibus) and two Hebrew translations (Sefer ha-Gvulim), but only a fragment of the original Arabic (Kitab al-Hudud) survived. That this book was widely read by Christian scholastics is evidenced by Thomas Aquinas’ and Albertus Magnus’ misattribution of Avicenna’s definition of truth to Isaac’s Book of Definitions (see Altmann &amp; Stern, Isaac Israeli, p. 59). This book is a collection of 57 definitions, the bulk of which are paraphrases and quotes (often without attribution) of various passages of al-Kindi. In some instances, he cites “the philosopher” who would be understood to mean Aristotle, but he is actually paraphrasing al-Kindi, who oftentimes purports to elucidate what Aristotle had written on a particular subject. Occasionally, his definitions derive from other possible sources such as Qusta ibn Luqa (a physician, mathematician, scientist and translator, lived 835–912 C.E.), Ammonius Hermiae of Alexandria (Fifth Century C.E.) and John Philoponus (490–ca. 570 C.E.). A few ideas expressed in this work are not attested in any other known sources. Yet, because of the book’s close adherence to al-Kindi, Stern hypothesizes that these deviations are often either quotation of definitions of al-Kindi which have been lost, or are misreadings of al-Kindi. One of the colophons at the end noted that the work is a “collection,” perhaps indicating that the material was not original, nor intended to be read as such. In another manuscript the colophon asserts that Isaac Israeli had written it, yet this same colophon contains spurious information—that Israeli was from Spain and that he had fathered children whom he hoped would study it (Altmann &amp; Stern, Isaac Israeli, p. 78), hence its claim that the treatise is an entirely original work of Israeli’s must not be regarded as authoritative.