Philosopher Timothy Williamson, on counterfactual conditionals, evidential roles, and ants:
Then the danger is that far too much will count as a priori. Long-forgotten experience can mold my judgment in many ways without playing a direct evidential role, for example by calibrating my skillful application of concepts and conditioning me into patterns of expectation which are called on in my assessment of ordinary counterfactual conditionals. How we know (25) may turn out to be quite similar to how many of us know (26):
(26) If two marks had been nine inches apart, they would have been further apart than the front and back legs of an ant.
Sense experience need play no direct evidential role in knowledge of (26). One can know (26) without remembering any occasion on which one perceived an ant, and without having received any testimony about the size of ants. The ability to imagine accurately what an ant would look like next to two marks nine inches apart suffices.
— From “Knowledge of Metaphysical Modality,” in The Philosophy of Philosophy, 167 (H/T noted philosopher Joshua Blanchard)
However, consider (27), contributed by me:
(27) If two marks had been one inch apart, they would have been further apart than the front and back legs of an ant.
It is likely that (26) can be reasonably concluded based on little knowledge of insects but an assumption that any species of ant couldn’t possibly be so large as to have legs nine inches apart. However, knowledge of (27) requires prior knowledge acquired only through scientific investigation of the 15-20 thousand species of ants (and their corresponding legs). In this case, “the ability to imagine accurately what an ant would look like next to marks one inch apart” does not suffice. What makes (26) different from (27)? Idk.
The largest ants have queens of greater than 1.5 inches in length (Antblog), likely quite a bit larger than many people would expect based on “sense experience”. Williamson’s point about (26) only works with facts involving preposterous comparisons. In fact, knowing that mantids can reach 12 inches long and the Hercules beetle can reach 7 inches long (Wikipedia), I only really accept (26) based on my knowledge (provided by scientific discovery) that the largest of the thousands of ant species found to date is only around 2 inches. Without such information, I could easily imagine a nine-inch ant. Scientific inquiry is perhaps the most general method by which one can know such facts about ant anatomy.
However, I was given the above quote out of context, so this seemingly immensely important objection may actually be irrelevant to Williamson’s overall purpose in using ants as an example.