A term may conform to the reality or not; that is, it may signify a kind of which there are examples in the universe of discourse or it may not. It is therefore not accurate to define a proposition as which is either true or false. A proposition should be defined as that which professes to be true, or assigns a logical value to itself. The Truth is defined as that logical value which a proposition assigns to itself. Whether or not there really is such value, whether there is any truth is a question, not of definitions, but of fact.
A proposition is a symbol in which the representative element, or reason, is left vague and unexpressed, but in which the reactive element is distinctly indicated. In driving along the road with a compagnon, he points at a house and remarks, “That is a pretty house.” It is a proposition. But I make a voyage to a distant island and repeat the remark, “That is a pretty house.” “What house,” asks my interlocutor. “Oh, a house far way.” He will be right in telling me that I do not enunciate anything at all, because I do not indicate what I am talking about.
Symbols are of three classes: terms, which call attention to things or quasi-things; propositions, which declare facts; and arguments, which profess to enlighten us as to the rational connections of facts or possible facts.
A Proposition is a Sign which distinctly indicates of what Object (called its Subject) it is the Sign, but which is left to determine what Interpretant it may.
From an early/discarded draft
…a proposition is not a single thing and cannot properly be said to have any existence. Its mode of being consists in its possibility. A proposition which might be expressed has all the being that belongs to propositions although nobody ever expresses it or thinks it. It is the same proposition every time it is thought, spoken, or written, whether in English, German, Spanish, Tagálog, or how. A proposition consists in a meaning, whether adopted or not, and however expressed. That meaning is the meaning of any sign which should signify that a certain iconic representation, or image, (or any equivalent of it) is a sign of something indicated by a certain indexical sign, or any equivalent thereof. To illustrate this, any sentence will answer.
…every proposition is capable of expression either by means of a photograph, or composite photograph, with or without stereoscopic or cinetoscopic elaborations, together with some sign which shall show the connections of these images with the object of some index, or sign or experience, forcing the attention, or bringing some information, or indicating some possible source of information; or else by means of an analogous icon appealing to other senses than that of sight, together with analogous forceful indications, and a sign connecting the icon with these indices. [—]
A proposition is the signification of a sign which represents that an icon is applicable to that which an index indicates.
A representamen is either a rhema, a proposition, or an argument. An argument is a representamen which separately shows what interpretant it is intended to determine. A proposition is a representamen which is not an argument, but which separately indicates what object it is intended to represent. A rhema is a simple representation without such separate parts.
An informant index is a proposition. For one cannot better define a proposition (as distinguished from the assertion whereby one assumes responsibility for its truth) than as a representation of which one part serves, directly or indirectly, as an index of its object, while the other part excites in the mind an image of the same object.
…Peirce regards a proposition, by which he means the substance of a judgment considered as abstracted from the assent to it or dissent from it, a symbol which has a part by which it separately indicates its object while it also signifies this object in another way. The genus is the dicisign…
This quote has been taken from Kenneth Laine Ketner's 1983 reconstruction of Peirce's 'Autobiography'. Ketner identifies the source as "variant pages" of the manuscript.
It is remarkable that while neither a pure icon nor a pure index can assert anything, an index which forces something to be an icon, as a weathercock does, or which forces us to regard it as an icon, as the legend under a portrait does, does make an assertion, and forms a proposition. This suggests the true definition of a proposition, which is a question in much dispute at this moment. A proposition is a sign which separately, or independently, indicates its object.
The peculiar characteristic of the proposition lies not in its possibly being false, but in its possibly turning out to be false, and this whether it has been positively held or merely by not suspecting the possibility of its denial. That is wherein all advancement and diffusion of knowledge consists.
Note that a proposition is nothing existent, but is a general model, type, or law according to which existents are shaped. [—]
Now a proposition consists of two parts, the predicate, which excites something like an image or dream in the mind of its interpreter, and the subject, or subjects, each of which serves to identify something which the predicate represents.
A sign which though calculated to produce belief, that is, though it has the character of possible assertion, is nevertheless not necessarily so intended, nor necessarily used toward that end, and which may even carry a mark to show that it is actually not so intended, is a proposition.
A Proposition is any product of language which has the form that adapts it to instilling belief into the mind of the person addressed, supposing him to have confidence in its utterer. [—] A Proposition is nearly the same as an “Assertion.” The distinction which I use the two words to mark is that an Assertion includes no more than it is the intention of the Utterer to declare, while the Proposition includes all that he does declare, which is inevitably considerably more than he intends.