Yes, Atticus had almost certainly mentioned Procilius in his previous letter to Cicero, which prompted Cicero's dismissive response. [1, 2, 3]

We can infer the exact nature of Atticus’s missing message based on the timeline and literary clues within the surviving text of Letters to Atticus: [1]

1. Atticus Recommended Procilius

In Letters to Atticus (2.1), written just shortly before the letter in question, Cicero announced his grand plans to write a major geographical and historical treatise ("The geographical work I had planned..."). Atticus, acting as an encouraging friend and literary advisor, appears to have written back suggesting that Cicero consult Procilius as a primary source for the project. [1, 2, 3, 4, 5]

2. Cicero’s Direct Rebuttal

When Cicero replies in Letters to Atticus (2.2), he is directly shooting down Atticus's reading recommendation: [1]

"unde multo plura didiceris quam de Procilio!"
("...There is a deal more to be learned from [Dicaearchus] than from Procilius.") [1]

3. The Missing Correspondence

Because the Letters to Atticus only preserve Cicero’s side of the conversation, Atticus’s original letter containing the mention of Procilius is entirely lost. [1]

This specific interaction highlights how the collection functions: it reads like a one-sided telephone conversation where we must reconstruct Atticus’s questions and book recommendations from Cicero's sharp reactions. [1, 2, 3]

If you would like to explore what Cicero’s planned geographical project looked like—or why he preferred the Greek philosopher Dicaearchus instead—just let me know!