A comment by James on a somewhat recent Languagehat post introduced me to a term used by Duns Scotus and the Scholastic philosophers: haecceity. Haecceity can be rendered as “thisness.” By contrast, quiddity constitutes “whatness.”
Haecceity captures the characteristics that distinguish a particular individual: “Socrates” is a man “who lived in Greece.” Whereas quiddity refers to the universal qualities that a thing shares with all members of its genus: a man is a “featherless biped.”1
As a naturalist, I am always switching focus back and forth between a bird’s (or plant’s, or…) haecceity and quiddity, either in the particular instance or in the abstract. Haecceity: what are the characters (field marks) that distinguish this species from others? Quiddity: what is its gist? if you’ve never seen one before, what does it look like?
Haecceity is captured by the textual descriptions in your field guide, as well as the “Peterson system” arrows pointing to field marks in the paintings. Quiddity is best represented by the composite photographs in Crossley or Kaufman field guides. New birders usually gravitate toward quiddity, and I’ll flip open my Peterson or Sibley to show them paintings of a bird we’re talking about (and maybe have just missed seeing).
And here’s another concept that perhaps the Scholastics didn’t grapple with: characteristics that distinguish one taxon from another in the context of a particular dichotomous key.
Maybe I should stop here before I write anything more that’s unschooled.
1Dang, I recently read something good about dinosaurs being featherless bipeds and I can’t find it again.