premise:Its first 80 pages are by a single copyist,but the remaining 20 pages are by three different copyists,which indicates some significant disruption.
conclusion:Since a letter in handwriting identified as that of the fourth copyist mentions a plague that killed many people in Florentine in 1148,Codex Berinesis was probably produced in that year.
reasoning: the author hypothesizes some significant disruption cause the fact"Its first 80 pages are by a single copyist,but the remaining 20 pages are by three different copyists". according to the letter of the fourth copyist, which indicates that a plague killed many people in 1148, the author take granted that this plague is actually "some significant disruption". This reasoning is unsounded unless other possible explanation for "some significant disruption" have been ruled out. Imagine that perhaps there are large numbers of plagues in that period and the fourth copyist just noted one of them.Hence, you can not make sure which plague contributes to the phenomenon in the argument. | 
