Before the printing press, books could be purchased only in expensive manuscript copies.
Conclusion (this demonstrates that)
There was a dramatic jump in the number of people who learned how to read in the years after publishers first started producing books on the printing press.
Evidence
Before the printing press, books could only be purchased as expensive manuscript copies.
The printing press produced books significantly less expensive than manuscript editions.
The public's demand for printed books was many times greater than the demand for manuscript copies.
Evaluate
This argument hinges on a causal claim that the increased demand for printed books is due to a surge in literacy rates after the printing press was introduced.
CURIOUS FACT: why was there a dramatic jump in the demand for books, once printed copies became available?
EXPLANATION: way more people had learned how to read, and so there were more people than before who wanted to buy books.
We could weaken this argument by suggesting that the increased demand for books was not because more people learned to read, but perhaps because those who already could read simply bought more books due to their lower cost. This presents an Alternate Explanation for the increase in demand that doesn't involve a rise in literacy.
Goal
Let's find an answer that provides an alternate explanation for the increase in demand for printed books that does not rely on an increase in the number of people who learned how to read, or one that hurts the plausibility of the author's story (makes it seem like literacy did not go up).