Duns Scotus links “immaterial” with “not [spatially] extended” (reminding us of Descartes):
(b) Another way in which this knowledge could be immaterial would be that it is not extended in any way. In this case much more is asserted than the fact that it is not organic. For although everything organic is extended inasmuch as it is received into something extended [viz. the organ], this is not the only reason. It would still be extended if it were received immediately by the composite as a whole, because the composite is itself extended.
– as quoted in Allan B. Wolter: Duns Scotus Philosophical Writings, p. 141
Though Aquinas does claim that
the word ‘body’ is used to denote a genus of substances from the fact of their possessing three dimensions”
– Thomas Aquinas: Summa Theologiae, First Part, Question 18, Article 2)
it seems that for mainstream scholastics being just material (and not also being a body) was not defined by extension and instead by being dependent on prime matter:
For most scholastic authors, in contrast [to Descartes], extension lacks any such significance. Since they tend to be quantity realists (Ch. 14), they are generally committed to the possibility of extensionless material substances (material substances that are, strictly speaking, not bodies [§16.6]).
– Robert Pasnau: Metaphysical Themes 1274–1671, p. 325
Duns Scotus wasn’t a Thomist, but he must have known that most scholastics did have no issues with the notion of unextended material objects (a modern example would be an electron, certainly material, which is speculated to be unextended). If he wanted to show that the soul is not extended, this does not seem to be a very important result, because it does not show the soul to be immaterial.
Did Duns Scotus believe that material things were extended?
If so, why? Did he give any arguments for this proposition?