Consider again the proposition “A true ⇒ (if A then B) OR (if A then not B)”. Take B as: “This rock is heavier than this pencil.” Now, assuming that the pencil does not lie in the weight range 1.5 plus/minus sigma, then this proposition must be true. And now, this statement is significantly more complicated than “A ⇒ ~~A”, and it implies that (under proper restrictions) you can make longer logical statements, and continuing further, statements which are no longer trivial and just a property of human language.
I’m a little confused by this example. The proposition
A ⇒ (if A then B) OR (if A then not B)
is a logical tautology. It’s truth doesn’t depend on whether “the pencil does not lie in the weight range 1.5 plus/minus sigma”. In fact, just the consequent
(if A then B) OR (if A then not B)
by itself is a logical tautology. So, I have two questions:
(1) Is there a reason why you didn’t use just the consequent as your example? Is there a reason why it wouldn’t “get to the heart” of your point?
(2) Just to be perfectly clear, are you claiming that the truth of some tautologies, such as A ⇒ ~~A , is “trivial and just a property of human language”, while the truth of some other tautologies is not?
Sorry, I caught that myself earlier and added a sidenote, but you must have read before I finished:
Side-note: I suppose these particular examples are all tautological so they don’t quite show the full richness of a logical system. However, it would be easy to make theorems, such as “if A AND C, then B” (where C could be specified similar to A or B.) Then we would see not only tautologies but also theorems and other propositions which are all encoded as we would expect from a typical logical system.
Edit: Or, sorry, just to complete, in case you had read that—the tautology does depend on whether the pencil lies in the range of 1.5 plus/minus sigma. If the pencil lies in that range, we can’t say B or ~B.
In answer to (1.), I’m not using the consequent because you identified the fact that the consequent can imply anything by logical explosion. I was referring to the “A=>~A” example not getting to the heart of the point because that example is too simple to reveal anything of substance, as I subsequently discuss.
In answer to (2.), I am not claiming that some tautologies are “less true”. I am just roughly showing how there is a gradation from obvious tautologies to less obvious tautologies to tautologies which may not even be recognizable as tautologies, to theorems, and so on.
First, I, at least, am glad that you’re asking these questions. Even on purely selfish grounds, it’s giving me an opportunity to clarify my own thoughts to myself.
Now, I’m having a hard time understanding each of your paragraphs above.
Or, sorry, just to complete, in case you had read that—the tautology does depend on whether the pencil lies in the range of 1.5 plus/minus sigma. If the pencil lies in that range, we can’t say B or ~B.
B meant “This rock is heavier than this pencil.” So, “B or ~B” means “Either this rock is heavier than this pencil, or this rock is not heavier than this pencil.” Surely that is something that I can say truthfully regardless of where the pencil’s weight lies. So I don’t understand why you say that we can’t say “B or ~B” if the pencil’s weight lies in a certain range.
In answer to (1.), I’m not using the consequent because you identified the fact that the consequent can imply anything by logical explosion. I was referring to the “A=>~A” example not getting to the heart of the point because that example is too simple to reveal anything of substance, as I subsequently discuss.
I didn’t say that the consequent can imply anything “by logical explosion”. On the contrary, since the consequent is a tautology, it only implies TRUE things. Given any tautology T and false proposition P, the implication T ⇒ P is false.
More generally, I don’t understand the principle by which you seem to say that A ⇒ ~~A is “too simple”, while other tautologies are not. Or are you now saying that all tautologies are too simple, and that you want to focus attention on certain non-tautologies, like “if A AND C, then B” ?
In answer to (2.), I am not claiming that some tautologies are “less true”. I am just roughly showing how there is a gradation from obvious tautologies to less obvious tautologies to tautologies which may not even be recognizable as tautologies, to theorems, and so on.
But surely this is just a matter of our computational power, just as some arithmetic claims seem “obvious”, while others are beyond our power to verify with our most powerful computers in a reasonable amount of time. The collection of “obvious” arithmetic claims grows as our computational power grows. Similarly, the collection of “obvious” tautologies grows as our computational power grows. It doesn’t seem right to think of this “obviousness” as having anything to do with the territory. It seems entirely a property of how well we can work with our map.
B meant “This rock is heavier than this pencil.” So, “B or ~B” means “Either this rock is heavier than this pencil, or this rock is not heavier than this pencil.” Surely that is something that I can say truthfully regardless of where the pencil’s weight lies. So I don’t understand why you say that we can’t say “B or ~B” if the pencil’s weight lies in a certain range.
My idea was that the rock weighs 1.5 plus/minus sigma. If the pencil then weighs 1.5 plus/minus sigma, then you can’t compare their weights with absolute certainty. The difference in their weights is a statistical proposition; the presence of the sigma factor means that the pencil must weigh less than (1.5 minus sigma) or more than (1.5 plus sigma) for B or ~B to hold. But anyways, I might concede your point as I didn’t really intend this to be so technical.
I didn’t say that the consequent can imply anything “by logical explosion”. On the contrary, since the consequent is a tautology, it only implies TRUE things. Given any tautology T and false proposition P, the implication T ⇒ P is false.
Sorry, “logical explosion” is just a synonym for “ex falso quodiblet”, which you originally mentioned. You originally pointed out that the consequent can imply anything because of ex falso quodiblet, when A is not true. That wasn’t my intention, so I added the A true qualifier.
More generally, I don’t understand the principle by which you seem to say that A ⇒ ~~A is “too simple”, while other tautologies are not. Or are you now saying that all tautologies are too simple, and that you want to focus attention on certain non-tautologies, like “if A AND C, then B” ?
It initially seemed too simple for me, but maybe you are right. My original thinking was that “A ⇒ ~~A” seems to mean merely that a statement makes sense, whereas other propositions seem to have more meaning outside of that context. Also, the class of tautologies between different propositions seems to generalize the class of tautologies with a single proposition.
… It doesn’t seem right to think of this “obviousness” as having anything to do with the territory. It seems entirely a property of how well we can work with our map.
I hadn’t really thought about this, and I’m not sure how important it is to the argument, although it is an interesting point. Maybe we should come back to this if you think this is a key point. For the moment I am going to move to the other reply...
I guess my original idea (i.e., the idea I had in my very first question in the open thread) was that the physical systems can be phrased in the form of tautologies. Now, I don’t know enough about mathematical logic, but I guess my intuition was/is telling me that if you have a system which is completely described by tautologies, than by (hypothetically) fine-graining these tautologies to cover all options and then breaking the tautologies into alternative theorems, we have an entire “mathematical structure” (i.e., propositions and relations between propositions, based on logic) for the reality. And this structure would be consistent, because we had already shown that the tautologies could be formed consistently using the (hypothetically) available data. Then physics would work by seizing on these structures and attempting to figure out which theorems were true, refining the list of theorems down into results, and so on and so forth.
I’m beginning to worry I might lose the reader do to the impression I am “moving the goalpost” or something of that nature… If this appears to be the case, I apologize and just have to admit my ignorance. I wasn’t entirely sure what I was thinking about to start out with and that was really why I made my post. This is really helping me understand what I was thinking.
Tell me whether the following seems to capture the spirit of your observation:
Let C be the collection of all propositional formulas that are provably true in the propositional calculus whenever you assume that each of their atomic propositions are true. In other words, C contains exactly those formulas that get a “T” in the row of their truth-tables where all atomic propositions get a “T”.
Note that C contains all tautologies, but it also contains the formula A ⇒ B, because A ⇒ B is true when both A and B are true. However, C does not contain A ⇒ ~B, because this formula is false when both A and B are true.
Now consider some physical system S, and let T be the collection of all true assertions about S.
Note that T depends on the physical system that you are considering, but C does not. The elements of C depend only on the rules of the propositional calculus.
Maybe the observation that you are getting at is the following: For any actual physical system S, we have that T is closed under all of the formulas in C. That is, given f in C, and given A, B, . . . in T, we have that the proposition f(A, B, . . .) is also in T. This is remarkable, because T depends on S, while C does not.
This looks somewhat similar to what I was thinking and the attempt at formalization seems helpful. But it’s hard for me to be sure. It’s hard for me to understand the conceptual meaning and implications of it. What are your own thoughts on your formalization there?
I’ve also recently found something interesting where people denote the criterion of mathematical existence as freedom from contradiction. This can be found on pg. 5 of Tegmark here, attributed to Hilbert.
This looks disturbingly similar to my root idea and makes me want to do some reading on this stuff. I have been unknowingly claiming the criterion for physical existence is the same as that for mathematical existence.
This looks somewhat similar to what I was thinking and the attempt at formalization seems helpful. But it’s hard for me to be sure. It’s hard for me to understand the conceptual meaning and implications of it. What are your own thoughts on your formalization there?
I’m inclined to think that it doesn’t really show anything metaphysically significant. When we encode facts about S as propositions, we are conceptually slicing and dicing the-way-S-is into discrete features for our map of S. No matter how we had sliced up the-way-S-is, we would have gotten a collection of features encoded as proposition. Finer or coarser slicings would have given us more or less specific propositions (i.e., propositions that pick out minuter details).
When we put those propositions back together with propositional formulas, we are, in some sense, recombining some of the features to describe a finer or coarser fact about the system. The fact that T is closed under all the formulas in C just says that, when we slice up the-way-S-is, and then recombine some of the slices, what we get is just another slice of the-way-S-is. In other words, my remark about T and C is just part of what it means to pick out particular features of a physical system.
Though the word “tautology” is often used to refer to statements like (A v ~A), in mathematical logic any true statement is a tautology. Are you talking about the distinction between axioms and derived theorems in a formal system?
I’m a little confused by this example. The proposition
A ⇒ (if A then B) OR (if A then not B)
is a logical tautology. It’s truth doesn’t depend on whether “the pencil does not lie in the weight range 1.5 plus/minus sigma”. In fact, just the consequent
(if A then B) OR (if A then not B)
by itself is a logical tautology. So, I have two questions:
(1) Is there a reason why you didn’t use just the consequent as your example? Is there a reason why it wouldn’t “get to the heart” of your point?
(2) Just to be perfectly clear, are you claiming that the truth of some tautologies, such as A ⇒ ~~A , is “trivial and just a property of human language”, while the truth of some other tautologies is not?
Sorry, I caught that myself earlier and added a sidenote, but you must have read before I finished:
Edit: Or, sorry, just to complete, in case you had read that—the tautology does depend on whether the pencil lies in the range of 1.5 plus/minus sigma. If the pencil lies in that range, we can’t say B or ~B.
In answer to (1.), I’m not using the consequent because you identified the fact that the consequent can imply anything by logical explosion. I was referring to the “A=>~A” example not getting to the heart of the point because that example is too simple to reveal anything of substance, as I subsequently discuss.
In answer to (2.), I am not claiming that some tautologies are “less true”. I am just roughly showing how there is a gradation from obvious tautologies to less obvious tautologies to tautologies which may not even be recognizable as tautologies, to theorems, and so on.
First, I, at least, am glad that you’re asking these questions. Even on purely selfish grounds, it’s giving me an opportunity to clarify my own thoughts to myself.
Now, I’m having a hard time understanding each of your paragraphs above.
B meant “This rock is heavier than this pencil.” So, “B or ~B” means “Either this rock is heavier than this pencil, or this rock is not heavier than this pencil.” Surely that is something that I can say truthfully regardless of where the pencil’s weight lies. So I don’t understand why you say that we can’t say “B or ~B” if the pencil’s weight lies in a certain range.
I didn’t say that the consequent can imply anything “by logical explosion”. On the contrary, since the consequent is a tautology, it only implies TRUE things. Given any tautology T and false proposition P, the implication T ⇒ P is false.
More generally, I don’t understand the principle by which you seem to say that A ⇒ ~~A is “too simple”, while other tautologies are not. Or are you now saying that all tautologies are too simple, and that you want to focus attention on certain non-tautologies, like “if A AND C, then B” ?
But surely this is just a matter of our computational power, just as some arithmetic claims seem “obvious”, while others are beyond our power to verify with our most powerful computers in a reasonable amount of time. The collection of “obvious” arithmetic claims grows as our computational power grows. Similarly, the collection of “obvious” tautologies grows as our computational power grows. It doesn’t seem right to think of this “obviousness” as having anything to do with the territory. It seems entirely a property of how well we can work with our map.
My idea was that the rock weighs 1.5 plus/minus sigma. If the pencil then weighs 1.5 plus/minus sigma, then you can’t compare their weights with absolute certainty. The difference in their weights is a statistical proposition; the presence of the sigma factor means that the pencil must weigh less than (1.5 minus sigma) or more than (1.5 plus sigma) for B or ~B to hold. But anyways, I might concede your point as I didn’t really intend this to be so technical.
Sorry, “logical explosion” is just a synonym for “ex falso quodiblet”, which you originally mentioned. You originally pointed out that the consequent can imply anything because of ex falso quodiblet, when A is not true. That wasn’t my intention, so I added the A true qualifier.
It initially seemed too simple for me, but maybe you are right. My original thinking was that “A ⇒ ~~A” seems to mean merely that a statement makes sense, whereas other propositions seem to have more meaning outside of that context. Also, the class of tautologies between different propositions seems to generalize the class of tautologies with a single proposition.
I hadn’t really thought about this, and I’m not sure how important it is to the argument, although it is an interesting point. Maybe we should come back to this if you think this is a key point. For the moment I am going to move to the other reply...
Little note to self:
I guess my original idea (i.e., the idea I had in my very first question in the open thread) was that the physical systems can be phrased in the form of tautologies. Now, I don’t know enough about mathematical logic, but I guess my intuition was/is telling me that if you have a system which is completely described by tautologies, than by (hypothetically) fine-graining these tautologies to cover all options and then breaking the tautologies into alternative theorems, we have an entire “mathematical structure” (i.e., propositions and relations between propositions, based on logic) for the reality. And this structure would be consistent, because we had already shown that the tautologies could be formed consistently using the (hypothetically) available data. Then physics would work by seizing on these structures and attempting to figure out which theorems were true, refining the list of theorems down into results, and so on and so forth.
I’m beginning to worry I might lose the reader do to the impression I am “moving the goalpost” or something of that nature… If this appears to be the case, I apologize and just have to admit my ignorance. I wasn’t entirely sure what I was thinking about to start out with and that was really why I made my post. This is really helping me understand what I was thinking.
Tell me whether the following seems to capture the spirit of your observation:
Let C be the collection of all propositional formulas that are provably true in the propositional calculus whenever you assume that each of their atomic propositions are true. In other words, C contains exactly those formulas that get a “T” in the row of their truth-tables where all atomic propositions get a “T”.
Note that C contains all tautologies, but it also contains the formula A ⇒ B, because A ⇒ B is true when both A and B are true. However, C does not contain A ⇒ ~B, because this formula is false when both A and B are true.
Now consider some physical system S, and let T be the collection of all true assertions about S.
Note that T depends on the physical system that you are considering, but C does not. The elements of C depend only on the rules of the propositional calculus.
Maybe the observation that you are getting at is the following: For any actual physical system S, we have that T is closed under all of the formulas in C. That is, given f in C, and given A, B, . . . in T, we have that the proposition f(A, B, . . .) is also in T. This is remarkable, because T depends on S, while C does not.
Does that look like what you are trying to say?
This looks somewhat similar to what I was thinking and the attempt at formalization seems helpful. But it’s hard for me to be sure. It’s hard for me to understand the conceptual meaning and implications of it. What are your own thoughts on your formalization there?
I’ve also recently found something interesting where people denote the criterion of mathematical existence as freedom from contradiction. This can be found on pg. 5 of Tegmark here, attributed to Hilbert.
This looks disturbingly similar to my root idea and makes me want to do some reading on this stuff. I have been unknowingly claiming the criterion for physical existence is the same as that for mathematical existence.
I’m inclined to think that it doesn’t really show anything metaphysically significant. When we encode facts about S as propositions, we are conceptually slicing and dicing the-way-S-is into discrete features for our map of S. No matter how we had sliced up the-way-S-is, we would have gotten a collection of features encoded as proposition. Finer or coarser slicings would have given us more or less specific propositions (i.e., propositions that pick out minuter details).
When we put those propositions back together with propositional formulas, we are, in some sense, recombining some of the features to describe a finer or coarser fact about the system. The fact that T is closed under all the formulas in C just says that, when we slice up the-way-S-is, and then recombine some of the slices, what we get is just another slice of the-way-S-is. In other words, my remark about T and C is just part of what it means to pick out particular features of a physical system.
Though the word “tautology” is often used to refer to statements like (A v ~A), in mathematical logic any true statement is a tautology. Are you talking about the distinction between axioms and derived theorems in a formal system?