Sentence:  The "Speculator" responded to the letter with a blend of commiseration and alarm: "this young gentleman's situation is truly pitiable. The <WSD>"cacoethes</WSD> scribendi" is certainly one of the most troublesome diseases of the mind; and when it thus totally possesses a man, I scarce know any madness equal to it" (51:20). The <WSD>"cacoethes</WSD> scribendi", which as "An Impartial and Candid Disquisition into the Case of Sporus", by A Lover of Truth and Impartiality (1755) explained, "is a hard Word for a Disease called in plain "English", the Itch of Writing," was a staple of literary satire (24). 
Meaning of the word marked with <WSD> tags:  Compulsion; mania.