1

I am reading a bit about the implicational propositional calculus and initially I was confused about the axiom system:

Axiom Schema 1: $P ⟹ (Q ⟹ P)$

Axiom Schema 2: $(P ⟹ (Q ⟹ R)) ⟹ ((P ⟹ Q)⟹ (P⟹R))$

Axiom Schema 3: $((P ⟹ Q) ⟹ P) ⟹ P$

Each one of these is a tautology; moreover, $P ⟹ (Q ⟹ P)$ and $((P ⟹ Q) ⟹ P) ⟹ P$ are both tautologies in $2$ variables. My initial confusion (which I still have not resolved and must point to a fundamental gap in my understanding) is that I am not sure why one of $P ⟹ (Q ⟹ P)$ or $((P ⟹ Q) ⟹ P) ⟹ P$ can't be derived from the other since they are both tautologies in $2$ variables.

In my attempts to figure out what my misunderstanding was I thought that maybe it had something to do with the fact that:

  • $((P ⟹ Q) ⟹ P) ⟹ P$ has $3$ instances of $P$, whereas $P ⟹ (Q ⟹ P)$ has only $2$

or maybe it had something to do with the fact that

  • $((P ⟹ Q) ⟹ P) ⟹ P$ has $3$ instances of $⟹$, whereas $P ⟹ (Q ⟹ P)$ only has $2$

And to test that theory I wanted to see if one of $P ⟹ (Q ⟹ P)$ or $Q ⟹ (P ⟹ P)$ is derivable from the other since they both are tautologies in $2$ variables with $2$ instances of $⟹$, $2$ instances of $P$, and $1$ instance of $Q$.

The problem is, I don't know how to go about showing that one is (or isn't) derivable from the other in the implicational propositional calculus.

Any insight into this issue will be greatly appreciated.

Nika
  • 727
  • A schema is an outline for axioms. 'P' in an axiom schema like (P⟹(Q⟹P)) stands for any formula. Technically each axiom that a schema can generate is a tautology. – Doug Spoonwood Jan 05 '20 at 14:26

3 Answers3

2

By 'being derivable from each other' I assume that you can still use Axioms 2 and 3, but use 'the other' as Axiom 1.

As such, $Q \to (P \to P)$ can be derived from Axioms 2 and 3 together with $P \to (Q \to P)$, i.e. the original Axiom 1:

\begin{array}{lll} 1 & (P \to ((P \to P) \to P) \to ((P \to (P \to P)) \to (P \to P)) &Axiom \ 2\\ 2 & P \to ((P \to P) \to P) & Axiom \ 1\\ 3 & (P \to (P \to P)) \to (P \to P)& MP \ 1,2\\ 4 & P \to (P \to P) & Axiom \ 1\\ 5 & P \to P & MP \ 3,4\\ 6 & (P \to P) \to (Q \to (P \to P)) & Axiom \ 1\\ 7 & Q \to (P \to P) & MP \ 5,6\\ \end{array}

However, $P \to (Q \to P)$ can not be derived from Axioms 2 and 3 together with $Q \to (P \to P)$. Here is why.

Suppose we have a $4$-valued semantics. That is, suppose that all variables take on one of three values $0$, $1$, $2$, or $3$. Now assume that the $\to$ operator works as specified by the following table with the value of the left-operand on the left, and the value of the right-operand at the top:

\begin{array}{c|cccc} & 0&1&2&3\\ \hline 0&1&1&1&1\\ 1&0&1&3&3\\ 2&0&1&1&0\\ 3&0&1&1&1\\ \end{array}

Thus, for example, $1 \to 2 = 3$

Let's see what happens with this semantics for the $\to$ to the statements of $Q \to (P \to P)$, $((P \to Q) \to P) \to P$, and $P \to (Q \to P)$:

\begin{array}{cc|c|rc|cll|rc} P&Q&P\to Q & Q \to & (P \to P) & ((P \to Q) & \to P) & \to P & P \to & (Q \to P)\\ 0&0&1&\color{red}1&1&1&0&\color{red}1&\color{red}1&1\\ 0&1&1&\color{red}1&1&1&0&\color{red}1&\color{red}1&0\\ 0&2&1&\color{red}1&1&1&0&\color{red}1&\color{red}1&0\\ 0&3&1&\color{red}1&1&1&0&\color{red}1&\color{red}1&0\\ 1&0&0&\color{red}1&1&0&1&\color{red}1&\color{red}1&1\\ 1&1&1&\color{red}1&1&1&1&\color{red}1&\color{red}1&1\\ 1&2&3&\color{red}1&1&3&1&\color{red}1&\color{red}1&1\\ 1&3&3&\color{red}1&1&3&1&\color{red}1&\color{red}1&1\\ 2&0&0&\color{red}1&1&0&1&\color{red}1&\color{red}1&1\\ 2&1&1&\color{red}1&1&1&3&\color{red}1&\color{red}0&3\\ 2&2&1&\color{red}1&1&1&3&\color{red}1&\color{red}1&1\\ 2&3&0&\color{red}1&1&0&1&\color{red}1&\color{red}1&1\\ 3&0&0&\color{red}1&1&0&1&\color{red}1&\color{red}1&1\\ 3&1&1&\color{red}1&1&1&3&\color{red}1&\color{red}1&3\\ 3&2&1&\color{red}1&1&1&3&\color{red}1&\color{red}0&0\\ 3&3&1&\color{red}1&1&1&3&\color{red}1&\color{red}1&1\\ \end{array}

We see that $Q \to (P \to P)$ and $((P \to Q) \to P) \to P$ always evaluate to $1$. As such, we can call them '$1$-tautologies'. But note that $P \to (Q \to P)$ is not a $1$-tautology.

Now, what about the statement $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$? We could do a $64$ row table, but we can reason this out more quickly.

First, since $\varphi \to \psi$ never evaluates to $2$, $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ will never evaluate to $2$.

Second, for the same reason that $\varphi \to \psi$ never evaluates to $2$, we know that $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ cannot evaluate to $3$. This is because the only way for it to evaluate to $3$ would be when $P \to (Q \to R)$ evaluates to $1$, and $(P \to Q) \to (P \to R)$ to $3$, meaning that $P \to Q$ would have to evaluate to $1$, and $P \to R$ to $3$, and that in turn means that $P$ would have to evaluate to $1$ and $R$ to $2$ or $3$. However, with $p$ being $1$, $P \to Q$ can only evaluate to $1$ if $Q$ evaluates to $1$, and with $R$ being $2$ or $3$, that means $Q \to R$ evaluates to $3$, and hence $P \to (Q \to R)$ would evaluates to $3$, rather than $1$. So, $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ cannot evaluate to $3$.

Finally, and again because $\varphi \to \psi$ never evaluates to $2$, the only way for statement $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ to evaluate to $0$ would be when $P \to (Q \to R)$ evaluates to $1$ or $3$, and $(P \to Q) \to (P \to R)$ to $0$, and the latter means that $P \to Q$ would have to evaluate to $1$ or $3$, and $P \to R$ to $0$, and that in turn means that $P$ would have to evaluate to $1$, $2$, or $3$, and $R$ to $0$. This means that $Q$ cannot evaluate to $0$, for otherwise $P \to Q$ would have to evaluate to $0$ as well. But if $Q$ evaluates to $1$, $2$, or $3$, then $Q \to R$ evaluates to $0$, and hence $P \to (Q \to R)$ would evaluate to $0$ as well, so that can't be either. So, there is no way for $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ to evaluate to $0$.

In sum: $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ will always to evaluate to $1$, and is therefore a $1$=-tautology.

OK, so using $Q \to (P \to P)$, $((P \to Q) \to P) \to P$, and $(P \to (Q \to R)) \to ((P \to Q) \to (P \to R))$ as the axioms, it turns out that they are all $1$-tautologies under this semantics.

Also, note that when applying Modus Ponens to any two $1$-tautologies, you will always get another $1$-tautology, because the only time that $\varphi \to \psi$ and $\varphi$ both evaluate to $1$ is where $\psi$ evaluates to $0$. So, applying Modus Ponens to $1$-tautologies, you can only get more $1$-tautologies.

However, we already saw that $P \to (Q \to P)$ is not a $1$-tautology. Therefore, $P \to (Q \to P)$ cannot be derived from the other three.

Bram28
  • 100,612
  • 6
  • 70
  • 118
  • But is $P ⟹ (Q ⟹ P)$ derivable from Axioms 2 and 3 together with $Q ⟹ (P ⟹ P)$? – Nika Jan 05 '20 at 02:20
  • @Nika Thinking about that right now ... – Bram28 Jan 05 '20 at 02:22
  • Your second line is missing a parenthesis Bram :) Solid proof otherwise. – Brevan Ellefsen Jan 05 '20 at 02:31
  • @BrevanEllefsen Quite so! :) – Bram28 Jan 05 '20 at 02:57
  • @Nika My suspicion is that $P \to (Q \to P)$ is not derivabl;e from the other three .. but no luck proving that so far. Here I describe a general method of proving non-derivability (= independence). Unfortunately, if there is any counter-model at all, there does not seem to be one with $3$ values. ... and I'm not looking forward to trying this with $4$. Got any programming skills yourself? – Bram28 Jan 05 '20 at 03:25
  • @Bram28 I'm going to try to give this a shot. – Nika Jan 05 '20 at 19:25
  • @Nika Got the little bugger!! – Bram28 Jan 05 '20 at 21:37
  • @Bram28 I possibly might have found one in $4$ values but I am not sure at all. I definitely need more practice at showing non-derivability – Nika Jan 05 '20 at 21:38
  • @Nika Well, I don't have a lot of practice with it either, actually. But note that providing a non-standard semantics isn't the only way ... as Doug Spoonwood shows, sometimes you can point to some purely syntactical property.... though this is typically harder to show. Indeed, I note that Dous's Answer only shows that $P \to (Q \to P)$ is not derivable from $Q \to (P \to P)$ alone ... he did not take into account Axioms 2 and 3. – Bram28 Jan 05 '20 at 21:44
  • @Nika Also, sorry, but I did not demonstrate the truth of my claims that the three statements are $1,2,3$-expressions in my Answer ... it's a good bit of work. I may have some time later to add this ... – Bram28 Jan 05 '20 at 21:45
  • @Bram28 Looking at your example, as well as some other examples of counter-models (I read through Łukasiewicz's 'The Shortest Axiom of the Implicational Calculus of Propositions') they all seem to have the diagonal of the matrix having $1$. The counter-model I attempted didn't have that, is that an immediate sign that it won't work? – Nika Jan 05 '20 at 21:46
  • @Bram28 This is the one I was working on: \begin{array}{c|cccc} & 0&1&2&3\ \hline 0&0&0&3&3\ 1&2&1&2&3\ 2&0&0&0&3\ 3&1&0&1&1\ \end{array} But I got stuck because under the interpretation $P = 1, Q = 2, R = 3$ I got that $Q ⟹ (P ⟹ P) $ evaluates to $0$ while $(P ⟹ (Q ⟹ R)) ⟹ ((P⟹Q)⟹ (P⟹R))$ evaluates to $1$. Which I thought might be an issue already... But I guess it just means that they are $0,1$-expressions! – Nika Jan 05 '20 at 21:47
  • @Nika Huh! No idea ... here is where my practice with these is definitely lacking myself ... but let me check your model ... please tell me: what is the semantical property that you believe all 3 axioms have, and that is preserved by MP, but that $P \to (Q \to P)$ does not have? – Bram28 Jan 05 '20 at 21:47
  • @Nika I added some further explanation and demonstrations. I also made a slight change to my previous model so it is all a little cleaner now. Hope this helps in your understanding of this method! – Bram28 Jan 06 '20 at 17:08
0

Axiom 3 is known Pierce's Law, which is the implication connective form of the Law of the Excluded Middle and is known to be unprovable from the deduction theorem alone (where we include Modus Ponens as a tautology along as part of the deduction theorem; Modus Ponens is also assumed in the Implicational Propositional Logic, so this is reasonable). Just because you have things we decide to be "tautologies" of a given number of variables doesn't mean you can immediately prove other "intuitive tautologies" of the same number of variables. Your system might not be strong enough.

To conclude here, note the Wikipedia Page for the Deduction Theorem includes proofs of both your Axioms 1 and 2 using only the Deduction Theorem. If Axiom 3 could be proved from Axioms 1 and 2 this would mean Pierce's Law could be proved by the Deduction Theorem, known to be false.

I cannot yet think of a quick way to show Axiom 3 does not imply Axiom 1. If you use classical logic and translate the axioms you will find they all say very different things, and there is no reason to suspect any of the three axioms are redundant.

  • Could you point me towards the proof that the LEM can't be proved from the deduction theorem? I think another part of my misunderstanding is that I am not sure in what sense $((P ⟹ Q) ⟹ P) ⟹ P$ is equivalent to LEM, although I have no doubt that it is equivalent to LEM. I also do not think that the axioms are redundant, but if someone asked me to prove that they are not redundant I would not know how to do that. – Nika Jan 05 '20 at 02:36
  • @Nika I've read it before, but can't trace down the source. Wikipedia mentions it on both the page on Peirce's Law and on the page on the Deduction Theorem (which I would highly recommend reading). This Stack Exchange Answer seems to point to sufficiency to form a complete system. Still researching a bit – Brevan Ellefsen Jan 05 '20 at 02:42
  • I have thought of a route you can take to prove this. Note that Peirce's Law implies the law of the Excluded Middle, so all we have to do is consider an intuitionist logic in which we can derive the first two axioms. However, it is well known that the full classical intuitionist logic can derive the Deduction Theorem, so we are done - we cannot prove Peirce's Law, else our system would not be intuitionist! This actually shows Peirce's Law is even stronger than axioms 1+2+intuitionist assumptions – Brevan Ellefsen Jan 05 '20 at 02:45
  • @Nika The law of the excluded middle is a disjunction. It's not derivable from the above three axioms without a definition. – Doug Spoonwood Jan 05 '20 at 14:29
0

The simplest way to look at this sort of problem is to look at the axiom systems through condensed detachment. An axiom system under Condensed detachment only produces theorems which are the most general theorems up to relettering of the variables given the axioms. A theorem A is more general than another theorem B if by using only substitution in A we can obtain B, but we can't use substitution from B to obtain A. For example, (P$\rightarrow$(P$\rightarrow$P)) is not a most general type of theorem in the implicational propositional, since both (Q→(P→P)) and (P→(Q→P)) are more general.

Suppose, (Q$\rightarrow$(P$\rightarrow$P)) is our only axiom. Alright, so we make some copy of (Q$\rightarrow$(P$\rightarrow$P)) and put that in for Q. Then we'll obtain (P$\rightarrow$P) as a theorem. All other theorems are substitution instances of (Q$\rightarrow$(P$\rightarrow$P) and (P$\rightarrow$P). (P$\rightarrow$(Q$\rightarrow$P)) is not one of those theorems. Thus, (P$\rightarrow$(Q$\rightarrow$P)) is not derivable from (Q$\rightarrow$(P$\rightarrow$P)).

Conversely, the theorems generated via condensed detachment with (P→(Q→P)) follow a pattern and always get longer. Basically, one can see what this pattern is as follows:

  1. (P→(Q→P))

  2. (A1$\rightarrow$1.)

  3. (A2$\rightarrow$2.)

and so on ad infinitum.

(Q$\rightarrow$(P$\rightarrow$P)) isn't on that list, and thus can't get proved.