Lewis's motivation for writing the book was in explaining to students of the work of previous centuries that the definition of a word that they already think they know (his dangerous sense, which he abbreviates D.S.) may yield a total misunderstanding of what the author meant to say. Those who have a large vocabulary are actually more likely to pick a wrong meaning because they can rationalize its enjambment. Some of the earlier meanings are only partially recalled in stock phrases, such as "world without end" employs the earlier use of the word "world" to mean 'age'.
The words studied are nature, in all its phrases, especially "human nature"; sad, which originally meant "heavy"; wit; free, with all its differences from slavery and villainy; sense, with its two meanings of perception and judgement; simple; conscience and conscious; world; and life; with also the phrase "I dare say!" examined. The details of the history of these seemingly straightforward words encompasses 300 pages.