Keywords: applied category theory, musing, polynomial functors, blog
Summary: A recollection of conversation with David Spivak, the creator of Plausible Fiction at ACT2026.
At ACT 2026, I spoke with David Spivak about his idea of Plausible Fiction. While it is still a very early concept, David’s IPAM Talk gave a very nice overview of the work. Based on my recollection and notes from our conversation, the way he suggested to think about a plausible fiction is like a flow chart or as the “mathematics of a self-fulfilling prophecy”. An example he gave was, “suppose there is a task you want to do such as, ’we will make a national highway for the United States.”’ Accordingly, the outcomes for each step taken to achieve this goal are effectively a height one tree on a polynomial (i.e. a corolla) which could be explored further in the textbook, Polynomial Functors: A Mathematical Theory of Interaction.
As I recall from my conversation with David, this process can be interpreted as the kleisli category of the free monad on Poly to your desired goal or outcome. Each of these corollas can be thought of as directed rooted graphs ascending upwards, where each branch coming from a node of this tree represents a decision path. You can then recursively add these corollas “upwards” as you continue progressing up the tree and branching out from each end of a branch on your graph. The whole tree itself may have arbitrary heights like a binary tree does and does not have to be balanced and certainly does not have to have a height of one. An important aspect of this branching is that these trees of decisions do not require any particular resolution. You can define subtasks as granularly or coarsely as required to denote and achieve a goal.
When considering examples of plausible fictions, the example of “make a national highway,” actually comes from US President Eisenhower. David mentioned how the concept of a plausible fiction most directly applies to and is most readily understood by a goal or desired outcome described by an authority or individual figure leading an effort. Such an entity would then define and direct the progress related to a specific goal.
It may be a surprise to note that President Eisenhower, was, in fact, no civil engineer. Instead, what Eisenhower did and what plausible fiction allows, is the ability to make a critiquable, shared vision. When Eisenhower declared his goal for a national highway, there was no exact path to achieve this goal. Yet, he had plenty of advisors and aides who could help contribute their expertise to fill in all the necessary pieces along the way to that goal. Similarly, plausible fiction allows individuals with different skillsets to help the creator of the plausible fiction describe and address the gaps of a fiction with more details. In a way, plausible fiction can be thought of as an invitation to actualization.
Moving from historical to personal examples, David pointed out that a personal anecdote I shared would qualify as a plausible fiction. This anecdote was, and is, my nebulous goal of “bringing and applying category theory to public health.” In particular, the reasonable detailed progression of:
And while these pieces are no longer fiction but reality, there is still much work to be done to achieve my long-term plausible fiction!
To conclude our discussion, another example we discussed was considering health systems. Thinking about sweeping, general improvements to a health system would be an underspecified plausible fiction and hard to take action on. The reason being is that it is hard to imagine what an “improved” health system would be as right now, the goal is too broad. However, building on this, a future discussion could consider from a health informaticist’s point of view of what could be a valid plausible fiction in a health system. Perhaps imagining something like, “I will define how to make health informatics research studies composable,” might be a fun place to start!
I would like to express thanks to David Spivak for the discussion and some clarifications about the concept of Plausible Fiction!
Zelko, Jacob S. Conversation about Plausible Fiction at ACT2026. https://jacobzelko.com/notes/aaa-0295/. October 26, 2026.