What is the difference between metonymy and synecdoche (confused by a paragraph in White's Metahistory)?

Question

I've been reading H. White's Metahistory and he uses a well-known example (sails for ships) to show the difference between metonymy and synecdoche, but it confused me:

A similar kind of representation is contained in the Metonymical expression "fifty sail" when it is used to mean "fifty ships." But here the term "sail" is substituted for the term "ship" in such a way as to reduce the whole to one of its parts. Two different objects are being implicitly compared (as in the phrase "my love, a rose"), but the objects are explicitly conceived to bear a part-whole relationship to each other. The modality of this relationship, however, is not that of a microcosm-macrocosm, as would be true if the term "sail" were intended to symbolize the quality shared by both "ships" and "sails," in which case it would be a Synecdoche. Rather, it is suggested that "ships" are in some sense identifiable with that part of themselves without which they cannot operate.

I know there are possible arguments for every trope, but I've been taught that the expression used in this way was a synecdoche based on the part-whole argument and not the shared quality, which, to me, seems more like the metonymic attribute and not a synecdochal relationship.

Evidently, I've only recently started delving deeper into cognitive linguistics so any help would be deeply appreciated!

Answer 1

The distinction can be pretty nuanced, but in this case, as with many others, you could look at the example of "fifty sails" both metonymically and synecdochally. When viewed metonymically, "sail" could stand for "ship", as it makes up a significant part that is very closely associated with the ship's ability to operate. Thus, the relationship is about the significance the sail represents, as it is mecessary for the ship to be able to move, not because a sail makes up part of a ship, although that is true too.

From a synecdochical point of view, the relationship between the two stems from the sail being a part of the ship, thus emphasizing a part-whole relationship in which the sail is anecdotal for the entire ship.

White interpreting "sail" as metonymical shows that his argumentation revolves not around the sail being a part that represents the whole, but the function of the sail and the importance that goes along with it. He is highlighting the functional association.

Then again, language isn't a fixed set of rules and categories but rather an infinite array of individual expressions and usages of utterances. Metonymy, synecdoche, and other rhetorical devices are just labels we place on patterns we've observed. In reality, there are no strict boundaries—just the fluid and dynamic way humans use language to convey meaning. All these distinctions are convenient fictions, tools we've created to make sense of the complex, ever-evolving nature of human communication. A lot of linguists need to move beyond that rigid, categorical way of thinking.