C. S. Lewis, claims in his The Discarded Image. An Introduction to Medieval and Renaissance Literature (Cambridge: Cambride University Press, 1964) on p. 10: "At his most characteristic, medieval man was not a dreamer nor a wanderer. He was an organizer, a codifier, a builder of systems. ... There was nothing which medieval people liked better, or did better, than sorting things out and tidying up. Of all our inventions I suspect that they would most have admired the card index."
He does, of course, mean a card index of the systematic sort, the kind whose electronic equivalent is the outliner (not the Luhmannian sort that is designed to leave systematic distinctions open as much as possible).
So, by extension, one might also argue that "medieval man" was an outliner. But this is clearly false, as outlines were invented only in the sixteent century. Petrus Ramus wrote 1543 in the Dialecticae partitiones: "Sett forthe shortly the some of the text, which thou hast taken in hand to interprete: next... porte thy text into a fewe heads that the auditor may the better retaine thy sayings: Thirdly ... intreate of every heade in his owne place with the ten places of invention ... and last ... make thy matter playne and manifest with familiar examples & aucthorities out of the worde of God ..."
Medieval thinkers probably would not have understood the intricacies of the card index, nor were they outliners. They were inveterate list makers. Most of these lists were flat. Many of them rather short. Some of them were even hierarchical, such as the three triads of angels, but most of them would have lacked the intricacy that is made possible by the graphical representation of outlines that can be found in Ramist texts.
Having said this, I admit that the difference between lists and outlines is fluid, and that Ramus's invention was therefore not without precedent.