Edward sends this interesting example:
Omnis homo est mortalis
Socrates is a man
Sokrates ist sterblich
Semantically valid, but not syntactically?
No, syntactically valid because the argument instantiates a valid argument-form, to wit:
Every F is a G
a is an F
Therefore
a is a G.
Validity is a matter of form. An argument is valid if it instantiates a valid argument-form. It is the form that is valid or invalid in the primary senses of these terms. The argument itself is valid or invalid in secondary senses. The argument inherits its validity from the form, so to speak. Or you could say that it is the validity of the form that is the ground of, and accounts for, the validity of the argument.
For me, and here is where Ed will disagree, a valid deductive argument such as the 'Socrates' syllogism above, is a sequence of propositions, not of sentences, that instantiates a valid argument-form.
A proposition is what a sentence in the indicative mood expresses. To be precise, a proposition is what is expressed by the tokening (whether by utterance, writing, or in some other way) of a sentence in the indicative mood. The following three sentences, each from a different language, can be used to express one and the same proposition or Fregean Gedanke (thought) :
Sokrates mortalis est.
Sokrates ist sterblich.
Socrates is mortal.
These three numerically different sentence tokens from three different languages express the same proposition when they are used to express a proposition. Sentences are linguistic entities. Propositions are extra-linguistic, and therefore not tied to particular languages as sentences are. Not tied in the sense that the same proposition can be expressed in different languages. Suppose that every English speaker is exterminated. Could it then be said that Socrates is mortal? Yes, though not in those words. One could say the same thing by uttering the corresponding German or French or Turkish sentence.
This is a reason to distinguish propositions from sentences.
Now glance back at Ed's example. It is linguistically hybrid. But logically it expresses the very same argument (sequence of propositions) that the following does:
Every man is mortal
Socrates is a man
Ergo
Socrates is mortal.
The argument expressed is syntactically valid because it is an instance of a valid argument-form.
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