Critics and audiences alike have feverishly puzzled over Hamlet's actions ever since he first distinguished between kin and kind, probably around 1600. As Stephen Greenblatt notes1, the problem of Hamlet’s motivation -- the "heart of his mystery" -- is not one that is raised casually by the text of the play. It is “the central focus of the entire tragedy” (p. 307), a question that the play itself repeatedly invites its audience to consider and to reflect upon.

One such way in which the play encourages its audience is through example: many of Hamlet's various characters spend much of their time on stage crafting independent theories about the rationale behind Hamlet’s behavior. However, these theories are each incomplete and tend to offer more information about these theorists than about the object of their interpretation. For instance, consider how an audience of the play is frequently invited to laugh at Polonius’ superficial understanding of Hamlet’s behavior and how it reflects his own too-obvious attempts at social climbing (“Still harping on my daughter!”). Polonius, however, is merely the character that has the most noticeable habit of misreading his own expectations into Hamlet’s actions. Gertrude has a similar tendency to interpret all of her son’s activities as the result of her “o’erhasty marriage” alone -- a too-narrow interpretation that many critics, including T.S. Eliot, can be forgiven for sharing, especially since her remarriage is a topic that Hamlet rants about in great depth. Rosencrantz and Guildenstern tend to find the stalled ambitions of a courtier in their former schoolmate’s behavior, whereas Claudius seems to be concerned with Hamlet’s motivation only so far as it reveals the degree to which his nephew is a potential threat. Ophelia, like her father, waits in vain for Hamlet to give her signs of affection, and Horatio would have little reason to think that Hamlet was concerned with anything more pressing than the commandment of the ghost. The First Gravedigger seems to think that Prince Hamlet, like that "whoreson mad fellow” Yorick, is simply insane without any need for explanation. And so on.

Each one of the motives ascribed to Hamlet by other characters is certainly active at many points in the play. Hamlet, however, compounds the issue by toying with the narrow and singular interpretations of his behavior that each of the other characters clumsily present to him. When confronted by Ophelia immediately after his “to be or not to be” speech, for instance, Hamlet rebukes her affections, but then tells her that he once did love her -- and then immediately retracts the remark. When Rosencrantz and Guildenstern confront him after the staging of the Mousetrap and artlessly demand to know what is motivating his “distemper,” Hamlet disingenuously responds that he “lacks advancement,” thereby taunting the pair with an ironic echo of their own earlier insistences:
HAMLET. Why, then ‘tis none to you; for there is nothing either good or bad but thinking makes it so. To me it is a prison.

ROSENCRANTZ. Why, then your ambition makes it one: ‘tis too narrow for your mind.

HAMLET. O God, I could be bounded in a nutshell, and count myself a king of infinite space, were it not that I have bad dreams.

GUILDENSTERN. Which dreams, indeed, are ambition; for the very substance of the ambitious is merely the shadow of a dream. (2.2.249-259)
This reflection of motive is bizarrely constant thoughout the play -- and it operates even where it trumps common sense. For instance, as Rebecca West notes2, Hamlet never disabuses Gertrude of the idea that it is her remarriage alone that has motivated his misbehavior. “In the course of over eighty lines” addressed to Gertrude in her chamber, notes West, Hamlet “devotes only three to a perfunctory mention of the fact that her present husband murdered her previous husband, and when she shows that she did not know that any crime had been committed he does not take the opportunity of enlightening her” (228). Similarly, instances where Hamlet taunts Polonius with casual mention of Ophelia are almost too numerous to list, and the potential avenger of King Hamlet’s murder seems to be constitutionally unable to stand in the same room with Claudius without provoking him needlessly. The trend is not only for other characters to project their own expectations onto Hamlet’s behavior, but also for Hamlet to reinforce these same projections whenever possible, even when it is to his explicit benefit not to do so. Ophelia may therefore be more right than she knows when she describes him as “th’ expectancy and rose of the fair state, / The glass of fashion and the mould of form” (3.1.148-9).

However, there is also an interesting side-effect of Hamlet's continuous reflection of the various theories about his behavior. By thus reinforcing the assumptions of the play’s other characters, Hamlet also defers the problem of his “true” motivation indefinitely. Given this, no single character besides Hamlet is ever in a position to pronounce definitively upon the root causes of his behavior.

Real human beings watching this play have a similar problem with our conceptions of Hamlet’s activities. Any audience to Hamlet naturally assumes that it is what all audiences to staged plays always are -- in a position of intellectual superiority over the characters that it observes. Although the frequency of his monologues suggests otherwise, this is demonstrably not the case with regard to Hamlet. As Stephen Booth skillfully demonstrates3 in “On the Value of Hamlet,” an audience to this play’s collective understanding continuously lags behind Hamlet’s erratic behavior. Booth instances several convincing examples of this trend, although the analysis that is most germane to a discussion of Hamlet’s ability to evade audience expectations is the one that William Empson provides4 in his “Up-dating Revenge Tragedy.” Empson’s essay reveals that each of Hamlet’s soliloquies, which an audience listens to in anticipation of revelatory information,
contains a shock for the audience, apart from what it says, in what it doesn’t say: the first in having no reference to usurpation; the second (“rogue and slave”) no reference to Ophelia, though his feelings about her have been made a prominent question; the third (“To be or not to be”) no reference to his plot or his self-criticism . . . the fourth (“Now might I do it pat”) no reference to his obviously great personal danger now that the King knows the secret5; the fifth (“How all occasions do inform”) no reference to the fact that he can’t kill the king now, or rather a baffling assumption that he still can; and one might add his complete forgetting of his previous self-criticisms when he comes to his last words (291).
Hamlet’s motivation is slippery, and, like the attempts of a Polonius, a Rosencrantz or a Guildenstern, any individual audience member’s ability to track it and to pin it down is inevitably deficient. Greenblatt is astute when he elsewhere describes the “strategic opacity” that Shakespeare employs to deepen the mystery of Hamlet’s behavior for an audience, only this only this doesn’t fully explain the depths of the irrationailty underpining Hamlet's motivation. That opacity that Greenblatt describes is, after all, a deeply constructed effect, and Shakespeare is exceedingly adroit at evoking an audience’s sense of wonder at Hamlet’s behavior via the simultaneous juggling -- and the adamant refusal to decisively settle upon any one -- of several possible motivating principles.

I submit that this juggling is a large part of what makes Hamlet such an unusually complex figure. As Michael Goldman notes6,
Hamlet is soldier, scholar, statesman, madman, fencer, critic, magnanimous prince, cunning revenger, aloof noble, witty ironist, man of the people, etc.; and he is regularly required to change from one role to another before our eyes or to maintain several--or a disarming mixture of several--at once (74).
Such a “disarming mixture” is what I am most interested in. In his previous plays, Shakespeare generally presents his characters’ motives explicitly, as though they adhere to a straightforward and internally consistent set of principles. In Hamlet, his title character’s behavior is erratic, irrational and inconsistent within the contexts in which that behavior is presented to an audience. The effect of this is spectacular: As Greenblatt suggests, it is generally agreed upon by most audiences that Hamlet’s character possesses an extraordinary psychological depth as well as an indefinable sense of mystery. Above everything, however, this effect is an illusion that Shakespeare has deftly and deliberately generated. At any given second in the play, Hamlet is potentially inspired by any of several motives, each of which might potentially contradict another -- as might be a real human being. However, Shakespeare almost never ascribes to Hamlet the motivating principle that is most appropriate for the advancement of his plot, which is precisely what a theater audience is always most concerned with. We in the audience are like Polonius in this regard: we expect that Hamlet will help to advance our minds toward some final purpose -- in our case, the conclusion of the play’s narrative. Like Polonius, we are repeatedly and decisively thwarted by his actual, irrational behavior, which says anything and everything but to the point that we are most after. And like Polonius, we rarely understand what is happening until well after the fact, if we ever understand at all.
References/Notes

1. Greenblatt, Stephen Will in The World. New York: W.W. Norton & Company, 2004.

2. West, Rebecca "The Nature of Will." Hamlet: A Norton Critical Edition (2nd Edition). Ed. Cyrus Hoy. New York: W. W. Norton and Company, 1992. 227-237.

3. Booth, Stephen "On The Value of Hamlet." Reinterpretations of Elizabethan Drama. ed. Norman Rabkin. New York: Columbia University Press, 1969

4. Empson, William "Up-dating Revenge Tragedy." Hamlet: A Norton Critical Edition (2nd Edition). Ed. Cyrus Hoy. New York: W. W. Norton and Company, 1992. 283-294.

5. It's actually more accurate to say that Hamlet thinks that that he is in great personal danger because Claudius knows his secret. In The Mousetrap, Lucianus is not just a character that, like Claudius, murders a king in a garden by pouring poison in his ear -- he is also the nephew of his victim, like Hamlet. The same thing is not necessarily true of his analogue in the dumb show, which Claudius sits through unperturbed. Claudius, then, could be responding exclusively to what he perceives as a veiled threat from Hamlet. If this were so, it would also negate the idea, taken for granted by Hamlet, that Claudius confirms his own guilt by abandoning the play.

6. Goldman, Michael "Hamlet and Our Problems." Shakespeare and the Energies of Drama. Princeton, N.J.: Princeton UP, 1972. 74-93.