The english sentence snow is white doesn't translate to the FOL sentence \forall . x S(x) -> W(x). There are two reasons for this:
- The sentence snow is white is a generic, and generics don't express universal quantification.
- snow is a mass noun, and FOL predicates can't capture the semantics of mass nouns.
But (2) isn't really relevant to your question, so let's focus on (1). Generics are sentences like the following:
- dogs are mammals
- jewelry is expensive
- ducks lay eggs
- people are over three years old
- primary school teachers are female
They intuitively express generalizations of some kind, but linguists haven't had a lot of success uncovering the details of how their semantics works. However, one thing that's clear is that they do not express universal or existential quantification. They don't express universal quantification because e.g. (4) and (5) are true, but (4') and (5') are not:
4'. all jewelry is expensive
5'. all ducks lay eggs
(4') is false because some jewelry is cheap (I once bought a necklace for a dollar) and (5') is false because male ducks don't lay eggs. Notice that the same thing holds for your sentence: (8) is true, but (8') is not (just look at some dirty snow on the side of the road):
- snow is white
8'. all snow is white
This shows that generics don't express universal quantification. It follows that the fact that (8) and (8'') are contradictory, but (9) and (9') are not, has less to do with translating sentences of propositional logic into FOL and more to do with the nuances of generics:
8''. snow is not white
- \forall x . Snow(x) -> White(x)
9'. \forall x . Snow(x) -> ~White(x)
It's also worth mentioning that generics don't express existential quantification because e.g. (6) and (7) are both false, but (6') and (7') are both true:
6'. some people are over three years old
7'. some primary school teachers are female
If you want to learn more about generics, you can check out Ariel Cohen's chapter in this handbook or Sarah Jane Leslie's SEP entry.
EDIT: I'm answering @falcon's followup questions here:
Question 1: "Thank you for your answer! So, what you are saying is the following? “S are P” does not necessarily translate to “All S are P” or to “Some S are P”, neither does “S are not P” necessarily translate to “All S are not P” or to “Some S are not P”. This is because “S are P” and “S are not P” are generics, and generics express neither universal nor existential quantification."
Question 2: "Generics don’t express quantification because a generic might be true and its corresponding quantified-statement false, and vice versa--your two examples: “Ducks lay eggs” is true, but “ALL ducks lay eggs” is false; “Primary school teachers are female” is false, but “SOME primary school teachers are female” is true."
Answer: Yes, but with one caveat. Generics don't express universal or existential quantification, but many linguists think that they do express a distinct third kind of quantification called generic quantification.
Question 3: "And if all of the above is correct, would the following also hold, do you think? On account of differences in truth-value between generics and their corresponding quantified-statements, the inference from any generic to its corresponding quantified-statement is not tautologous. Because of this untautologousness, no generic is guaranteed any particular quantifier: generics tend thereby to be ambiguous in regards to quantification, and we must supply the missing quantifier through recourse to context in natural language."
Answer: No, I don't think that's right. An expression is ambiguous if there are multiple, distinct interpretations associated with it. In such a case, you should be able to paraphrase each interpretation and find scenarios in which only one of them is true. For example, the sentence all of the students failed an exam is ambiguous because it has exactly two distinct interpretations associate with it. And we can paraphrase them:
First interpretation: there is an exam that every student failed
Second interpretation: every student failed some exam or other
And finally, we can identify situations in which only the second interpretation is true (e.g. there are two students and two exams and the first student passed the first exam, but failed the second, while the second student failed the first exam, but passed the second).
However, this is not the case for generics. i.e. It's not the case that the sentence snow is white means all snow is white in some contexts, but some snow is white in other contexts. (just try thinking of such contexts!). Rather in every context, the sentence seems to have a single interpretation which simply is not equivalent to all snow is white or some snow is white.
With that said, here's something interesting that's not directly relevant to your question: the same surface syntactic structure can be used to express existentials and generics. For example,
1 a. racoons are mammals.
b. racoons are eating our trash
2 a. a cell phone is a portable means of communication
b. a cell phone is sitting on the table. do you know who it belongs to?
(1a) and (2a) are generics, but (1b) and (2b) are existentials. There are also empirical tests for telling them apart. Existentials are upward entailing, but generics are not. And the predicate in an existential must be a stage level predicate, but the predicate in a generic must be an individual level predicate (caveat: this isn't entirely true for indefinite singulars). But this isn't directly related to your question.
Question 4: "My error was in failing to account for this ambiguity, an ambiguity arisen from, as you put it, “the nuances of generics”."
Answer: I mostly answered this already, but no, I don' think that's right. I think the sentence snow is white is unambiguous. Your error was to interpret it as a universal when it's really (unambiguously) a generic.