Editor's note: John Locke's An Essay Concerning Human Understanding is available on this site.
In the modern era, Descartes placed the locus of self in the res cogitans, a thinking thing. Locke expands this definition of self also to include the memories of that thinking thing (II, xxvii, 24). For Locke, one's memories provide a continuity of experience that allows one to identify himself as self-same, i.e., as the same person over time (II, xxvii, 9). This theory of personal identity allows Locke to justify a defense of accountability. That is, since one is self-same one can be held accountable for past behavior (II, xxvii, 26). As we soon shall see, however, this theory is not without difficulties.
Locke's solution to the problem of personal identity consists of equating the self with the total set of memories available to that consciousness. That is, the thoughts and actions he can recall exhaust a person's identity. Only "as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person" (II, xxvii, 9). An agent that identifies these past thoughts and actions as deriving from its consciousness is, for Locke, self-same (II, xxvii, 25).
If, as Locke claims, these memories constitute a person's entire identity, then their accuracy has profound implications for personal accountability. For if one's memories are distorted, one's self-concept will be equally distorted; and if personal identity consists merely in one's memories, then accountability for the malicious intent of behavior requires one's acquiescence--otherwise the attribution constitutes an injustice (II, xxvii, 26). In other words, if memories are distorted, then Locke's theory of personal identity creates difficulties for his conception of accountability.
It is important to note, however, that Locke did not view consciousness as actively distorting memories. Nowhere in our selections from the Essay does Locke claim that memories are anything other than an accurate copy of ideas (II, x, 2). One is left to assume that Locke considers recalled memories passive, i.e., in no way biased or constructed by the agent. Since, for Locke, our memories are accurate, we can be held accountable for our past behavior (II, xxvii, 26). The central justification of personal accountability for Locke, then, is the accuracy of one's memories. My experience tells me, however, that in some cases memories do not provide an accurate conception of self. 
Take, for example, a case familiar to most professors. I first encountered this phenomenon in a hallway discussion among psychology students just after our professor returned our graded midterm examinations. My peers and I were complaining about the professor's strict and arbitrarily applied grading criteria. Each expressed his displeasure at the "injustice" of the professor's policy. After several minutes of impugning the professor's character the group dispersed. At the time I agreed with the group that we were the victims of a crazed professor bent on destroying our grade point averages. After some time passed and I had an opportunity to review the professor's comments on my test answers, I thought them reasonable and my grade appropriate.
Several days later, in contrast, a classmate once again expressed his view that the professor had "screwed us" on the midterm's grading. This classmate's memory excluded his poor performance on the test, and retained the professor's "mistreatment." Instead of taking an objective look at the facts of the case, he allowed his disappointment with his low grade to alter his recollection of the event. After the professor returned the test my classmate had a choice between taking a blow to his self-esteem by accepting his poor performance, or abandoning reality by blaming the professor. He chose the latter. The threat to his self-esteem resulted in his abandonment of the facts concerning his test performance.  His memory became not only distorted, but biased in his favor. His memory was distorted for the purpose of retaining his self-esteem.
If the professor explains to the student that the grade is appropriate and blame lies with his poor test answers, the student is likely to reject that claim and instead feel that for a second time he is the victim of a malicious professor. At this point his memory is not only inaccurate, but also is resulting in a shirking of responsibility and anger at being held accountable for what he perceives as not his fault.
This in not, however, the only manner in which memories can be distorted. Often people rationalize their behavior by attributing some kind of virtuous intent to their dishonorable acts.  These rationalizations create false memories of past events.
Consider the case of a nineteen-year-old boy, James, whose only remaining parent, his father, owns a successful business. James' father, a man who values altruistic behavior, has not decided whether to leave his business to James or Eric, James' brother. The father must decide quickly because inoperable cancer will soon take his life. James, eager about the possibility of owning his own business at only nineteen years of age, suddenly volunteers in a big brother program at the local YMCA. James knows he will have to reduce significantly the number of hours he spends practicing his favorite sport, tennis, but volunteers anyway. Bewildered by his son's out of character behavior the father asks James about his sudden interest in helping others. James contends that the YMCA program is the first to catch his attention and he doesn't mind sacrificing the pleasure of tennis practice to help the poor kids in need. That is, he claims that selfless service is the motive for his out of character behavior.
James, who learned from his father that altruistic behavior is the highest good, would be disgusted with himself if his reason for volunteering was "tainted" by selfish motives. That is, if his motive for volunteering was to please his father and thus increase his chances of inheriting the family business, he would consider his behavior immoral. James instead convinces himself that his intent is purely altruistic. If asked several years later about his volunteer work at the YMCA he remembers only his selfless service to the children. In this way, an act he would consider vicious if he remembered its true motive, i.e., his desire for his father's business, becomes in his mind a virtuous act "untainted" by selfishness. Nowhere in his memory do the real facts, i.e., his real motives, exist. All that remains is the rationalization.
The rationalization has convinced James that his behavior followed from only good intentions. His memory holds onto the rationalization and expunges the vicious intent--thus creating a distorted memory of the reason for volunteering. His skewed memory justifies his past evil acts and results in a warped self-concept.  His present false self-concept, then, follows from past instances of looking at his behavior through "rose colored glassed"; i.e., it follows from past instances of placing his desire to see himself as a good person above accepting who he really is. 
James' memory is not only distorted, but biased towards a goal. The goal is to hold onto an inflated, albeit false, self-concept. Similarly, in our first example my classmate held onto a false memory of his performance on the midterm for the purpose of avoiding the threat to his self-esteem.
While these are rather extraordinary examples, they illustrate how memory can be distorted by an active processing of experience that pursues a goal above the accurate perception of facts. Whether this goal is an inflated self-concept, enhanced self-esteem, or some other purpose, it is clear that memories are not passive. Contrary to Locke's theory, then, memories are not always accurate copies of ideas. 
According to Locke's doctrine, furthermore, one can be held accountable only for actions that one can remember. That is, one's responsibility ends with the total set of memories one can revive into one's conscious mind. One's "personality extends itself beyond present existence to what is past only by consciousness, whereby it becomes concerned and accountable, owns and imputes to itself past action" (II, xxvii, 26). One's accountability, then, extends no further than one's memory.
Holding someone accountable for actions he can not remember, for Locke, is the same as unjustly making him miserable. That is:
[T]o receive pleasure or pain, i.e., reward or punishment, on account of any such action is all one as to be made happy or miserable in its first being, without any demerit at all. For supposing a man punished now for what...he could be made to have no consciousness at all, what difference is there between that punishment and being created miserable. (II, xxvii, 26)Thus, punishing someone for behavior he has no recollection of is, to him, equivalent to punishing him for actions he never performed; for the state of the person who cannot remember his behavior is the same as the state of the man who never committed the act, viz. he is ignorant. In both cases, under Locke's theory of personal identity, the injustice is the same.
It is no less unjust, if Locke is correct, to hold someone responsible for actions about which he has a distorted memory. That is, if his memory of behavior X consists of his admirable intent and the righteousness of act X, then to punish him for X is itself unjust.  His biased memory makes it impossible to punish him without also acting unjustly; for he finds your punishment incongruent with his laudable behavior. Your punishment does not match his skewed memory.
If we adhere to Locke's theory of personal identity, then persons can be held accountable only for actions they remember, as they remember them--regardless of the accuracy of these memories. In cases of distorted memory, any other approach to accountability results in injustice. If we wish to avoid injustices, then not only must persons be exempt from responsibility for behavior they cannot remember, they must not be punished for evil actions their distorted memory labels "praiseworthy.”  In other words, punishment for previous evil acts requires the identification of those acts as vicious by the agent's memory. Otherwise, that punishment is, for him, equivalent to being created miserable--which Locke, as stated earlier, rejects as unjust (II, xxvii, 26).
While I concede that Locke probably would consider this account of responsibility incorrect and repugnant, I think his theory of personal identity, combined with the fact that memories are active and thus sometimes inaccurate, leads to this conclusion.
[Note 1] I use the concepts personal identity and self-concept interchangeably. I'm not sure whether a distinction can be drawn within Lockean terminology. If so, I do not see it.
[Note 2] Self-esteem is the result of a person's sense of his own self-worth and his confidence in the efficacy of his mind.
[Note 3] A rationalization is a false justification utilized for the purpose of hiding one's real motives from oneself or others.
[Note 4] This, of course, assumes a standard of value against which actions are judged. The nature of that standard is not important for my purposes. The example, however, utilizes altruism as the standard of value.
[Note 5] Holding James morally accountable is difficult because of the impossibility of discovering his intent. Since we don't have a window into the motives of others, knowledge of their intent seems impossible. Also, the example assumes a theory of accountability that holds intent as a crucial factor in making moral judgments.
[Note 6] If the understanding is passive (II, xxvii, 25) and recalled ideas are nothing more than an accurate copy of ideas acquired through the understanding, then memories are also passive (II, x, 2).
[Note 7] I'm assuming here that the parties involved accept the same moral code.
[Note 8] How to avoid a liar's taking advantage of this loophole in accountability is a question I can't answer.