The relation between uneasiness and desire in the chapter on power in the Essay (chapter XXI of book II) is at first glance unclear. The reader is told at one point that desire is uneasiness (II.xxi.32, 251 in the Nidditch edition), but at another the implication is that the instead of being the same thing as uneasiness, instead desire is what causes uneasiness (II.xxi.46, 262). Resolution of this tension is important to understanding Locke’s theory of the will, since uneasiness is taken to be the immediate cause of the will producing action (II.xxi.29, 249, II.xxi.31, 251). I will attempt here to explicate Locke’s concepts of uneasiness and desire in such a way as to highlight this tension in his theory, and will offer a speculation as to the theoretical motivation which may have prompted Locke to offer the account in such a way as to make it susceptible to this tension in the first place. But I don't have a solution on his behalf.
The role Locke assigns to uneasiness in his theory of the will is clear from II.xxi.29; what moves the mind to engage its general power of directing, or will, is some uneasiness (249). Uneasiness is the immediate cause of all action in Locke’s theory. In explaining why he has abandoned the position of the first edition of the Essay, that it is the greater good that moves the will, Locke indicates that one reason weighing in the favor of his newer account is that uneasiness is present to the mind, but the greater good is absent, and it would be in violation of the laws of nature to have an absent cause (II.xxi.37, 254). Thus, it is clear that Locke’s theory relies on causal mechanisms; the will directing action in the attempt to alleviate an instance of uneasiness follows causal rules. His theory relies on a causal model for the production of uneasiness as well, and Locke differentiates between two types if uneasiness based on their different causes (II.xxi.57, 271-2). First there is uneasiness which is the product of causes outside our power, like bodily pains from disease, injury, or the rack. These act “for the most part forcibly on the will” and will lead men to act against what they had in consideration determined was the best course of action. Such extreme disturbances will come to occupy the whole mind and prevent an agent’s ability to act with full consideration and will serve to ameliorate the responsibility for actions done in the heat of passion or the pain of injury (II.xxi.53, 267-8).
The other type of uneasiness is the product of desire for an absent good; this is the uneasiness which leads to the will provoking action that is the result of judgments about what will best lead to happiness (II.xxi.57, 268). On this picture, then, we would act in order to satisfy our desires for absent goods, which are determined to be worthwhile by our judgment except in cases of extreme disturbance or discomfort. Desire would thus be a technical term for Locke, reserved for cases in which it is a specific absent good toward which an agent’s consideration is directed. The mind’s judgment that some thing would be productive of happiness naturally leads to a desire for that good. Desire then functions to stir up uneasiness in an agent when the mind has used judgment to determine that some absent state of affairs is a good or will be conducive to happiness. Desire is thereby linked to our judgment in a way unavailable to us in uneasiness of the first type. We have the power, through “due consideration” of a particular good to raise our desire for that good to the level needed to provoke in us the uneasiness necessary to action. Without this, our wills are under the determination of the uneasiness most pressing upon us; the balancing that the mind can do being limited to determining which uneasiness should be addressed (II.xxi.46, 262).
This neat picture, however, is made problematic by two other features of Locke’s discussion of the relation between desire and uneasiness. The first problem is that at one point Locke claims that desire is uneasiness (II.xxi.32, 251), which cannot be maintained unless uneasiness can be a cause of itself, since as we have seen he considers desire to be a cause of one of the types of uneasiness. But Locke seems not to recognize this problem; he claims that desire is a specific form of uneasiness, one that is an uneasiness of the mind for want of some absent good. All bodily pain is an uneasiness, but Locke claims that this uneasiness is always joined to a desire, presumably a desire for the absent good which is the alleviation of the pain (II.xxi.31, 251). This problem could be fairly easily solved, however, by claiming that Locke is really just speaking loosely when he claims that desire is uneasiness, that what he means is that each instance of uneasiness is produced by a desire for an absent good; that each instance of uneasiness is accompanied by a desire. On this reading, we could avoid ascribing to Locke the view that uneasiness is a cause of itself. There is some textual evidence that suggests this reading. Locke indicates in II.xxi.39 that anger, fear and shame are correlated with uneasiness, and that each has desire as an accompanying it as well (257). Locke in this section also indicates that he believes that it is not possible to have uneasiness without a desire.
This solution produces a second problem, however, in that it breaks down the boundary between the two types of uneasiness which Locke had erected; Locke had distinguished the types of uneasiness by their causes, but if it turns out that each instance of uneasiness is the product of a desire, then there is only one type of uneasiness, not two. But Locke sought to distinguish between the two in part to explain why it is sometimes understandable that someone in extreme duress might act against their judgment of what would be most productive of happiness by positing a form of impingement on the will by uneasiness that circumvents the judgment altogether. His contention that all uneasiness is caused by a desire for an absent good, which is connected to judgment, undercuts this explanation.
What is unclear, given the difficulties just sketched, is why Locke may have felt compelled to view the relation between desire and uneasiness in this way in the first place. Part of the problem is that Locke is determined to provide a causal theory of the will, with natural philosophy as a model. He argues that there can be just one cause of our willing an action; over-determination is something to be avoided in a theory of the will (II.xxi.36, 254). Locke also insists that uneasiness, rather than the desire for an absent good must be what immediately moves the will, since the uneasiness is present, but the good is absent. Locke cannot countenance the causal efficacy of an absent thing; this would be a violation of natural law (II.xxi.37, 254). Only present causes can have effects. What Locke does not seem to consider, however, is that he could claim that though a desire may be for a good which is not present, that desire itself is present. He seems to be confusing the idea of desire with the content of it, and assuming the content of the idea is what is causally efficacious.
This confusion, if it is extended to the idea of uneasiness, may help explain the fact that Locke approaches uneasiness as if it had intentional content in the same manner as desire does. Instead of regarding uneasiness as an unspecified feeling of lack for which desire provides content via judgment (or some such formulation), Locke holds that instances of uneasiness are particular ideas about particular things. He talks at some points of “uneasinesses” in the plural (II.xxi.35, 253; II.xxi.37, 255) or of “an uneasiness” (II.xxi.31, 251), which is suggestive that he doesn’t regard the phenomenon of uneasiness as a general, content-free feeling (like we today might regard general sadness or depression), but instead looks at it as if each instance of uneasiness is directed toward some specific lack it compels the will to address. This specific content would allow it to act as a cause of which the will’s production of an action would be an effect; the content of the uneasiness is present in the mind, whereas the content of the desire is absent. But if each instance of uneasiness has a specific content, it can only be the faculty of judgment that provides the direction to the will for the best route to the alleviation of the uneasiness (since Locke has given us no alternative faculty for translating uneasiness into will-directed action). This is problematic, however, since uneasiness has been defined so as to be disconnected with judgment. Considerations like these may have motivated Locke to claim that all uneasiness is accompanied by desire, which is connected to judgment in such a way as to enable the will to see clear to an action that will result in an alleviation of the uneasiness. The complications surrounding desire and uneasiness in Locke’s theory of the will seem to be the result of ascribing content to uneasiness in order to allow uneasiness to have causal powers. This ascription, however, is only necessary because Locke has confused the presence of an idea with its content, leading him to conclude that desire itself cannot prod the will to action, since the content of desire is absent.