To Say Nothing of the Dog Themes
The concept of time travel clearly invites social commentary and, as in her previous works, Connie Willis delivers, granting readers a humanistic vision of humanity whose central values, beliefs, and behaviors remain unchanged over time.
One of the central thematic issues at work, coinciding with the social concern of clarifying our relation to the past, is that of preservation. Willis examines several different facets of the question how, given limited resources, we decide what is worth preserving from the past. Lady Schrapnell decides Coventry Cathedral is worthy of being rebuilt and is willing to pour money into that project; however, at times it seems that Lady Schrapnell's abrasive personality undermines the value of what she is doing as an unsympathetic character who often functions almost as an antagonist. Lady Schrapnell does not seem like a character whose value judgments the reader can trust.
At the same time, other alternatives for the cathedral such as turning it into a shopping center make the project less objectionable, and showing the building's deep emotional value to different characters from the time of its building and throughout history reinforces the probability that Lady Schrapnell's goal is not inherently wrong.
Other issues of preservation are raised by the discussion of time travel. One of the "laws" of time travel is that nothing significant can be brought forward from the past— it is impossible to bring forward the Mona Lisa because it already exists once at present, and the same is true for other treasures. However, one time-traveler accidentally breaks part of the law, revealing a whole new corollary to this rule, and raising once again the question of "significance" and value.
Another theme that surfaces most noticeably in the interactions with Professor Peddick and his nemesis, Professor Overforce, is the question of what determines historical outcomes. The aptly named Overforce holds that natural forces acting on populations determine history; Peddick staunchly maintains the primacy of individuals. At the end, Overforce capitulates. Ironically, however, Ned and Verity discover that their actions have, to a great extent, been guided by the continuum, which itself is an impersonal natural force. The final word on the subject seems to be that details so small they are never noticed have the greatest effect, a thesis Willis develops in further detail in her earlier novel Bellwether.