 In an important paper, Bobzien (2021) develops an inductively strong argument to the conclusion that Frege adopted ideas by the Stoics, as the latter were described in Carl Prantl’s four volume History of Western Logic (Geschichte der Logik im Abendland, 1855–1870) and in Diogenes Laertius’ Lives of the Philosophers (Vitae Philosophorum, 2nd century CE). The Stoic ideas described in these two works originate with Zeno of Citium, Cleanthes, Chrysippus of Soli, Diogenes of Babylon, etc., and so both works are ‘secondary’ literature.  The relevant reports in Diogenes’ Lives (which are largely quoted in Prantl) are among the closest we have to the source material for Stoic writings on language and logic, while Section VI of Volume 1 of Prantl’s Geschichte is a 95-page summary of Stoic views.  Bobzien notes how widely read Prantl's work was, and how Frege's training left him well-qualified to understand the many quoted passages in Greek and Latin in his work. She then compiles evidence to the conclusion that Frege borrowed heavily (and without attribution) from these works. This evidence is cumulative, for there is no smoking gun.  We can't point to any dog-eared copies with marginalia in Frege’s own hand, or any copies of works by Prantl and Diogenes that can be traced to Frege’s library, or any records of Frege checking out the works by Prantl and Diogenes from the Jena University library.[8]