So you can come up with a logic where modus ponens holds, and come up with a logic where modus ponens does not hold. Which would mean that if you wanted to find The Logic Of All and Only Common Principles (tm), you'd need to jettison modus ponens. Since it is not a common principle, since two logics disagree on whether it is a theorem. — fdrake
Just for extra detail - how easy it is to come up with logics that disagree on theorems is a good argument for nihilism if you agree, with a stipulated logical monist of a certain sort, that there is only one entailment relation which all of these logics ape.
The cases approach allows us to say more about what logical nihilism amounts to: it is the view that for any set of premises Γ and conclusion φ whatsoever, there is a case in which every member of Γ is true, but φ is not. — Russell
I underlined "any" and "there is a case" above to highlight something about their scope of quantification. What collection is being quantified over? It must generically include arbitrary cases, premises, conclusions etc. IE, "complete generality" in a manner that allows the arbitrary representation of statements in formal languages. It's thus a metalinguistic notion with respect to any object formalism, it lays beyond and out with them.
It's, furthermore, a semantic notion:
Henceforth I’ll assume the interpretations approach to logical consequence, on
which logical nihilism is the view that for every principle of the form Γ |= φ there is an interpretation of the non-logical expressions in Γ and φ such that every member of Γ comes out true but φ does not. Such an interpretation would be a counterexample to the principle. If it turns out that there are no such counterexamples, and that on every interpretation of those non-logical expressions on which each member of Γ is true, φ is also true, then the principle will be a logical law, and nihilism will be false.
The turnstile with two lines above means that Russell wants to find counterexamples to principles through interpreting the logic, which is a way of finding a "syntactically appropriate" mappings from its symbols to other objects - like propositions to truth values - to see in what conditions the proposed principle holds. Mucking about with interpretations like that is what makes the kind of logical nihilism she's playing with a semantic argument.
On the interpretations view Γ|=φ is true iff whatever (syntactically appropriate) interpretation is given to the non-logical expressions in Γ and φ, if every member of Γ is true, then so is φ. For example, if our argument is P a, a = b P b, then the interpretations approach says that the argument is valid iff there is no interpretation of P, a and b (assuming we are treating = as logical) such that P a and a = b are true, but P b is not. Models are understood as offering us different interpretations of the non-logical expressions, and hence if we find a model in which P a and a = b is true but P b is not, the principle is not true. On the interpretations conception then, logical nihilism is the view that for every argument, Γ φ, there are interpretations of the non-logical expressions in Γ and φ which would make every member of Γ true, but φ not true
So what Russell is doing, when she's finding counterexamples, is taking "syntactically appropriate" expressions, throwing them into a formalism, then evaluating them in that formalism through an interpretation. If she can find an expression and an evaluation that fit the rules of the logic that is also a counterexample to one of its candidate principles, then it's not a principle of the logic for all expressions in it - and so is not a logical law.
So the sense of "complete generality" also allows Russell to consider variations over interpretations and the relationship of interpretations with syntactical elements of languages - it's thus a highly metalinguistic notion. Which is not surprising, as the Logic Of All And Only Universal Principles would need to have its laws apply in complete generality, and thus talk about every other logical apparatus in existence.
Which is an incredibly, incredibly strong thing to want. It's practically alchemical, one must have in mind a procedure in which the complexities and ambiguities of natural language, every inference, can be stripped, dissolved, distilled into gold. The true atoms of rationality. The story hooks in the book of divine law. In some respects it's even stronger than the petty desire to take the intersection of all logics, at least that has a precedent in each logic. And you need to claim that this holy book of divine order is spoken in one voice, the true semantical derivation symbol of the cosmos, that admits no quibbling, sophistry or perversion.
Or you could refuse the above notion and take the path Russell does, by applying metalinguistic restrictions to the space of interpretations of a theory. As in, "yes, we know the Liar blows this logic up, so let's just say
for all bivalent φ", hence the method from proofs and refutations, lemma incorporation, in which a system is mapped to another system with an additional lemma in order to constrain its space of syntactically valid interpretations.
In formal terms, the latter is what distinguishes
@Leontiskos's sophist from someone who finds good counterexamples, someone who finds good counterexamples ensures that they are syntactically valid - that is, obeys all and only the stipulated rules, both intended and written. If you can jam something between the intention and the written word, while playing by all the rules stipulated, you've shown that the conceptual content of the formalism does not reflect the intended object. Or alternatively the intended object is the wrongly represented in the formalism, conceived in a confusing or inopportune manner etc.