Structure of logical argument is based on :

Concept: Every argument has two separable aspects — its form and its matter. The form is the arrangement of premises and conclusion (the pattern of inference); the matter is what the individual statements happen to assert about the world. Formal validity is the property that belongs to the form: an argument is formally valid when the conclusion follows necessarily from the premises purely by virtue of that arrangement, whatever content is substituted for the terms. This is why the skeleton or structure of an argument is settled at the level of form.

Application: Take the pattern "All A are B; x is an A; therefore x is a B." Substituting A = men, B = mortal, x = Socrates gives a familiar argument. Now substitute A = cats, B = birds, x = Tom: "All cats are birds; Tom is a cat; therefore Tom is a bird." The premises are false, yet the conclusion still follows necessarily from them, because the pattern of inference is untouched by the substitution. The structural backbone survives even when every statement in it is factually wrong — so what fixes the structure cannot be the factual accuracy of the statements.

Cross-check by contrast:

• Material truth is a property of individual propositions, not of the pattern joining them. It decides whether an argument is sound (valid form plus true premises), which is a further question asked after the structure is already fixed.

• Linguistic expression is the verbal clothing of the argument. Restating it in different words, or translating it into another language, leaves the inference pattern unchanged, so the wording cannot be what the structure rests on.

• Aptness of examples affects how persuasive or inductively well-supported a claim looks. Illustrations sit alongside an argument rather than forming the premise-to-conclusion link inside it.

Result: the structure of a logical argument rests on formal validity — the necessity with which the conclusion follows from the premises by virtue of the argument's form. (Material truth is what an argument needs in addition to formal validity to be sound.)