# Is this a valid proposition?

Consider following two sentences.

1. $x^2 = 1.$
2. Today is Thursday.

The first statement can't be a proposition. because the truth of (1) depends on the value of $x$. For some values of $x$ it is true (for $-1$ and $1$) and false for others.

We can apply the same reasoning for (2) (can we?) and conclude that it is not a valid proposition either as the truth of (2) depends on what day is $\text{Today}$.

A student pointed out that (2) could be a valid proposition if the intention was to state it for just one day i.e. today. I argued that if it were the case, then the proposition should have been more precise (by including the date may be ?)

My question is that , in the form (2) is currently written can it be considered a valid proposition ?

And if it is not valid, can it be written in a form without removing the word "Today" so that it becomes a valid proposition?

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No up vote, no down vote, no comment... Is this question really that bad ? Please comment if you need additional info or clarification. – Mohammad Yaseen Jan 2 '14 at 12:36
I would suggest looking at the topic of indexicals in philosophy of language. It's a totally different question from, say, the difference between an open formula and a sentence in a first order language, for the reason that it's natural language. Syntax is insufficient to tell you whether it's a proposition. – Malice Vidrine Jan 4 '14 at 20:30

Actually, they all can be proposition. The issue lie in what you count any of these as variable.

Look at this:Interpretation

If you let "today" and "x" be constant, then yes both of them are indeed proposition. The fact that they can varies is because of interpretation.

If you don't let them be constant, then they are not proposition.

In other word, whether they are or not depends on what are constant and what are not, which is up to you.

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So, 'validity' of the proposition depends on a particular interpretation ? – Mohammad Yaseen Jan 4 '14 at 9:50
My understanding is that proposition should be un-ambiguous and must have well-defined truth value. Am I missing something ? – Mohammad Yaseen Jan 4 '14 at 9:52
A valid formula is true under ALL interpretation. Now if you consider "proposition" in the philosophical sense, then yes maybe there is no such thing as interpretation, and a formula could in fact consist of actual object. But in maths, it's always symbol, meaningless symbol, that only get assigned by interpretation. Which is why, for example, we know that we cannot prove the parallel postulate from the rest of the Euclidean axioms: implication of the conjunction of these axiom to the postulate is not a valid formula, under a radical interpretation where "line" are interpreted as circle arc. – Gina Jan 4 '14 at 10:01
"in the form (2) is currently written can it be considered a valid proposition ?" Please correct me if I am wrong. You're suggesting that, it all depends on whether we allow Today and x to be fixed (well-defined) or not. If they are not variable, then the proposition is valid ? – Mohammad Yaseen Jan 4 '14 at 10:10
Oh no I'm not suggesting that. It all depend on whether they are variable or not. Let's say for example $S$ is a function, $1$ is a constant, and $E$ is a 2 place predicate. Then you have the first sentence being $E(S(x),1)$. Now I have not specify whether $x$ is a variable or not. If $x$ is a variable, then it's a free variable (no quantification), so not even a well-formed formula. If $x$ is a constant, then yes it is a well-formed formula, but not valid because you can pick an interpretation where it's false. – Gina Jan 4 '14 at 10:22

Short answer without reference to "first order logic" or "modal logic": Even $P$ and $Q$ are valid propositions in the appropriate context, hence there is no reason why your two sentences shouldn't be valid propositions. Note that I interpret "valid proposition" in the sense of "admissible proposition". You wonder about the value of $x$ here. The typical context in "first order logic" is that $x$ has a fixed value, but this often confuses people (because such a proposition is rarely useful in isolation). In other contexts like equational reasoning, it's also possible that $x$ is implicitly universally qualified (i.e. the proposition makes a statement about all possible $x$).

In the appropriate context, both of your sentences can be valid propositions. Let's start with $x^2 = 1$, because it's already stated in a "sugared" formal language. Let's assume its "desugared" formula to be $x\cdot x = 1$. Then it will be a valid proposition for a first-order language containing a constant "$1$" and a binary operator "$\cdot$". Here I interpret "valid proposition" as being member of the corresponding "formal language", i.e. in the sense of "admissible proposition". So the proposition could be false for some interpretation, but it would still be valid.

You wonder about the value of $x$ here. In the context of first-order logic, the value of $x$ is fixed as part of the interpretation. In the context of equational reasoning or universal algebra on the other hand, the equation (we don't usually talk about propositions in this context, only about equations and clauses) is only true (sadly, the correct word in this context would be "valid") if it is true for all possible values of $x$. Anyway, the point is that the formal semantics often allows free variables, and takes care to assign them an appropriate meaning in the interpretation.

Your second sentence "Today is Thursday" would only be used as an example for a proposition in the context of modal logic. In modal logic, we have both an interpretation and a collection of worlds, and we evaluate the truth of a proposition for a specific world. So the interpretation would give meaning to the words "Today", "Thursday", as well as to the connective "X is Y". The possible dates would also be fixed by the interpretation as the possible worlds (and how these possible worlds are related to each other by the modal operators), but the actual date/world where you evaluate this proposition is a part of the evaluation context, and not part of the interpretation. And the propositions are true or false with respect to a specific evaluation context, which includes and an interpretation and a specific world.

One might imagine designating a specific world as the actual world as part of the interpretation, but I don't know whether this is commonly done. From the texts on model logic I read so far, I have seen it once or twice, but it seems more common not to do this.

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To add some background, I am having a course in Discrete Maths and Logic. This is my first encounter with this subject, So I am actually not able to interpret your answer. Could you explain in some simpler terms + Could you recommend a good book ? – Mohammad Yaseen Jan 4 '14 at 16:44
If you don't know "first order logic" (also known as predicate calculus), then my answer is not appropriate for you, and I doubt that "simpler terms" would help in this case. In that case, please forget immediately that I even dared to mention "modal logic". Even if you know "first order logic", it's better not to care too much about the "modal logic" part, because it's more of interest for philosophers. I could recommend you excellent books about logic in German. For English literature, Peter Smith wrote the excellent Teach Yourself Logic: A Study Guide! – Thomas Klimpel Jan 4 '14 at 19:36
@MohammadYaseen I added a short section at the start that doesn't assume familiarity with "first order logic". Let me also add that the Stanford Encyclopedia of Philosophy is an excellent online source for many logic topics not covered in most introductory logic textbooks. It has no entry for "first order logic"/"predicate calculus", but the following entry on the history of proof theory contains interesting informations, and at least touches upon many subjects related to first order logic. – Thomas Klimpel Jan 4 '14 at 20:20