Othello By Shakespeare : Note on "The Motive-Hunting of Motiveless Malignity"

The famous phrase, "The motive-hunting of motiveless Malignity," occurs in a note Samuel Taylor Coleridge wrote in his copy of Shakespeare, as he was preparing a series of lectures delivered in the winter of 1818-1819. The note concerns the end of Act 1, Scene 3 of Othello in which Iago takes leave of Roderigo, saying, 

"Go to, farewell. Put money enough in your purse,"

and then delivers the soliloquy beginning 

"Thus do I ever make my fool my purse."

Here is Coleridge's note: 

The triumph! again, put money after the effect has been fully produced.--The last Speech, the motive-hunting of motiveless Malignity--how awful! In itself fiendish--while yet he was allowed to bear the divine image, too fiendish for his own steady View.--A being next to Devil--only not quite Devil--& this Shakespeare has attempted-- executed--without disgust, without Scandal!-- (Lectures 1808-1819 On Literature 2: 315) 

Coleridge's phrase is often taken to mean that Iago has no real motive and does evil only because he is evil. This is not far from what Coleridge meant, but he almost certainly wasn't using the word “motive" in the same way as it's now used. We use it to mean “an emotion, desire, physiological need, or similar impulse that acts as an incitement to action" ("Motive"). This definition equates “motive" and “impulse"; Coleridge, however, thought the two quite different. He makes this distinction in an entry he wrote for Omniana, a collection of sayings assembled by his friend Robert Southey and published in 1812. Here is what Coleridge wrote:

119. Motives and Impulses.

“It is a matter of infinite difficulty, but fortunately of comparative indifference, to determine what a man's motive may have been for this or that particular action. Rather seek to learn what his objects in general are!--What does he habitually wish? habitually pursue?--and thence deduce his impulses, which are commonly the true efficient causes of men's conduct; and without which the motive itself would not have become a motive. Let a haunch of venison represent the motive, and the keen appetite of health and exercise the impulse: then place the same or some more favourite dish, before the same man, sick, dyspeptic, and stomach-worn, and we may then weigh the comparative influences of motives and impulses. Without the perception of this truth, it is impossible to understand the character of Iago, who is represented as now assigning one, and then another, and again a third, motive for his conduct, all alike the mere fictions of his own restless nature, distempered by a keen sense of his intellectual superiority, and haunted by the love of exerting power, on those especially who are his superiors in practical and moral excellence. Yet how many among our modern critics have attributed to the profound author this, the appropriate inconsistency of the character itself!” (Shorter Works and Fragments 1: 310) 

Thus Coleridge asserts that Iago's motives (in our sense) were his “keen sense of his intellectual superiority" and his “love of exerting power." And so Iago's malignity is “motiveless" because his motives (in Coleridge's sense) -- being passed over for promotion, his suspicion that Othello is having an affair with his wife, and the suspicion that Cassio is also having an affair with Emilia -- are merely rationalizations.