Ergativity

[Matt Longhorn]

Reading a new book on the voice system in post-classical Greek. https://www.amazon.co.uk/Voice-Mood-Linguistic-Approach-Essentials/dp/1540964868/ref=sr_1_1?dchild=1&keywords=voice+greek+mathewson&qid=1635680095&qsid=261-6541831-7080941&sr=8-1&sres=1540961435%2CB008DUSU1A%2CB08XZDSNGB%2CB06XJPYZ9F%2C142342252X%2CB07KKF864Z%2C1617741620%2CB00CFI9AY2%2C0793570069%2C0853609594%2CB07JLFBVGX%2C157560616X%2C1847729746%2CB09CL6WTQT%2C0786682175%2CB07JLQXRD8

Trying to get my head around the concept of ergativity that Mathewson sees as key in the analysis of voice and getting confused.
On page 30 he writes
More concisely, “Is the process brought about from within, or from outside?” That is, in an ergativity-based interpretation of a clause, the notions of causality and agency play focal roles. According to Halliday, if the action is self-engendered, with no reference to an outside cause, the clause is nonergative. If the process has an external cause in its clause, and it is brought about by an agent, then the clause is ergative.
And then on page 31 referring to the sentence “the boat sailed” he writes
In this second sentence there is no specific reference to an agent. The action, then, is seen as self-engendering, occurring from within, with no reference to an external cause. Hence it is ergative.

Having a seriously dumb moment, but is he contradicting himself?
I read Halliday and Matthiessen’s chapter on this in Hallidays Introduction to Functional Grammar and came away with the understanding that an ergative representation of a subject is one without an external cause. So in Halliday's example “the lion chased the tourist” he sees this as an ergative/non-ergative contrast. According to this reading Mathewson’s second quote is correct (assuming he contradicting himself, which I allow is more likely to be my misunderstanding of him)

[Daniel]

(Note: I'm most familiar with these terms from the perspective of linguistic typology, i.e. how some languages mark cases using nominative-accusative systems, and others ergative-absolutive.)

I see why this appears confusing, and I agree. But the footnote there seems helpful. If I'm interpreting this correctly, this is because sail is considered to be an active kind of verb, as if the ship is choosing to sail itself, rather than someone sailing the ship. This relates to another concept, of split intransitivity, where (in some languages) some intransitives are marked like ergatives/nominatives and some like absolutives/accusatives (either label works, because the distinction is now semantic, not collapsed, in intransitives), depending on how active the verb is.

A typical example of split intransitivity is an active verb like "jump" vs. a passive verb like "fall". I believe that passage is suggesting that "sail" is more like "jump".

(There's another complex layer here related to metonymy or meronymy, where the "ship" refers to its crew/captain making decisions. If we said "The captain sailed." that would be more obviously an active sentence.)