The Merchant of Venice, though ostensibly a comedy, is one of the more serious plays in the comedic genre. The Taming of the Shrew is far more humorous and light hearted, but it is not without its lessons. The specific lessons vary greatly depending on one's interpretation of the play, especially in performance, but one key lesson that most of the female characters fail to learn is the advantage of working in tandem with their husband. Petruchio manages to win a substantial amount of money through his new wife Kate's quick obedience; she has learned through the course of the play to at least give the appearance of docility and subservience, which the other women lack -- they have failed to learn anything from her transformation, seeing no problems in themselves form the outset. This failure costs them some cold, hard, cash.

It is in Julius Caesar, however, that Shakespeare most clearly and explicitly illustrates the principle of a refusal to learn from others that Douglas Adams puts forth. Again, there are several small examples of this willful ignorance throughout the play, but the clearest and most substantial case is that such lies at the very center of the play's plot. Caesar is murdered because he has attained too much power; the other Senators fear that he will have himself crowned emperor and turn the democracy...

In murdering Caesar, however, these men -- especially Cassius -- are brining more power unto themselves, and ultimately face the same fate that Caesar faced, and largely for the same reasons. Neither Cassius nor Brutus would have died had they not assisted in the conspiracy against Caesar, but had instead learned form the dangers of seeking too much power and found a way to remove Caesar from it without appearing -- or very consciously trying -- to grasp it for themselves. The fact that ambition leads to ultimate destruction is one of the most basic plots in dramatic tragedy, and yet the men that inhabit Rome during the time of Julius Caesar fail to recognize this significance in their own actions.
There are many examples in the real world, outside the world of literature, that also express the truth of Adams' assertion that humans seem highly resistant to learning from each other. Politicians and CEOs are caught again and again making the same mistakes, people resume speeding minutes after seeing someone else receiving a ticket on the roadside, and unhealthy habits continue despite reading about heart attacks and other issues. Each individual seems to consider themselves specially able to avoid these common dangers, but life and literature shows us that they are incorrect.