In act five (scene three) of Macbeth, a doctor has been called to examine Lady Macbeth. She has been walking in her sleep, rubbing her hand, and speaking to herself. Worried that something may be seriously wrong, a doctor has come to examine her at the castle.
When Macbeth asks the doctor how his wife is doing, the doctor replies
Not so sick, my lord,
As she is troubled with thick coming fancies,
That keep her from her rest.
Here, the doctor is basically telling Macbeth that her "illness" is something he cannot treat. Essentially, her mind is ill; given she is "not so sick."
Macbeth tells the doctor to cure her and questions if the doctor cannot treat a "mind diseased." Macbeth seems to think that the memory which ails her can simply be removed from her brain. The doctor tells Macbeth that "Therein the patient must minister to himself."
What the doctor is essentially saying is that only Lady Macbeth can cure herself. She is the only one who can remove her guilt--no one else can. The doctor knows that mental illnesses are not something which can be cured (at this point in history).