In the opening exchange in Othello, Iago criticizes the recently promoted Cassio because he knows nothing about war other than what he has read in books and does not have Iago’s actual experiences in battle. The origin of the passage clearly comes from one underscored by North in his own Dial of Princes, in which the battle-hardened Hannibal levels the same criticisms against “the book soldier,” Phormio. Both Hannibal and Iago even use the peculiar word “Certes” (surely) and repeat many of the same lines.
In North’s passage, Hannibal complains that Phormio presumes to “know more in matters of war by that he hath read in books than doth Hannibal,” even though Phormio “never saw man of war in the field, nor understood” the true details of battle. Similarly, Iago complains about Cassio’s “bookish theoric” for he is one, “That never set a squadron in the field,/ Nor the division of a battle knows/ More than a spinster.” As Hannibal contrasts Phormio’s learning in peaceful cultural centers (“beautiful schools of Greece”) with his own experience in the “bloody fields of Afric” and fighting “dangerous wars … in Italy as in Spain;” Iago stresses that Cassio is a student from the cultural center of Florence while he himself had fought in the fields “At Rhodes, at Cyprus and on other grounds Christian and heathen.” As Hannibal complains that Phormio “hath seen not one thing with his eyes” of war; Iago says, “I of whom his eyes had seen the proof” of war. Hannibal dismisses the “book-soldier’s” “prating“; while Iago dismisses the “bookish” Cassio’s “prattle.”
In Nepos Lives, North also describes this same contrast as “theoric” vs. “practise,” which is precisely the antithetical pairing that Iago uses as well to describe the same idea. A search of Early English Books Online for just a few of these parallels confirms that the very passage that North had marked is indeed the same one that would inspire the original playwright of Othello.