During the late 19th century or early 20th C.S Peirce defined scientific theories as sets of propositions and made some interesting assertions about them. Who was the first to define scientific theories as sets of statements or propositions?
Who was the first to define scientific theories as sets of statements or propositions?
Such artificial definitions of scientific practice may be a relatively recent phenomenon going back to the beginning of the 20th century, and as such, there may be no publication of such a definition per se, but is understood implicitly since pre-Socratic notions of philosophy.
I suspect that no one has defined scientific theories per se, because historically scientists were the natural philosophers, and there was no strong division between the domains of philosophy, philosophy of science, and science in the modern sense. As such, practicing scientists since Galileo Galieli, who mathematized science' would have taken it as obvious that theories were collections of complete thoughts that proceeded by inferences across natural language and mathematics. Allegranza's invocation of Euclid's axiomatic method in the comment section goes back to organized displays of natural language. There is no likely publication that recognizes an absolute distinction between the theories of Parmenides and the modern scientific theory. (Of course, if I'm wrong, I'd like to know!)
While, the use of sentences to create theories goes back to the pre-Socratics and up to Charles S. Pierce, perhaps America's greatest philosopher, he may have anticipated the modern sense of scientific theory, as Bertrand Russell didn't become aware of his work until the 1920's. So, the formal rigorization of scientific theories as set-theoretic propositions in mathematical logic, while given credit to Bertrand Russell and then the logical empiricists contributed much to the use of language and mathematics to the philosophy of science and the received view may have been anticipated independently by Francis Bacon, William Whewell, or Charles S. Pierce. Today, philosophers of science frequently consider theories as collections of model-theoretic structures under the name semantic scientific theories.
According to the Encyclopedia of Philosophy, Volume 9, page 413, entry "Theories and Theoretical Terms":
In the early- and mid-twentieth century, philosophers of science, under the influence of Bertrand Russell's work in philosophy of language and philosophy of mathematics, attempted rationally to reconstruct scientific knowledge by representing scientific theories with the powerful conceptual tools provided by the formal theory of languages.
and a little later:
The syntactic view of theories (also called the received view) was developed by Rudolf Carnap, Ernest Nagel, David Hume...
Now, according to Blackwell's A Companion to the Philosophy of Science, entry "Theories", p.515:
Here the term "classical view" of scientific theories will be used in place of the expression "received view"... [which] had its origins in Europe, particularly the German-speaking regions... Its original proponents were self-proclaimed "scientific philosophers," such as Rudolf Carnap and Hans Reichenbach, who founded what later became logical empiricism.