Alternative Ways to Approach Propositional Calculus

There exist several ways to approach propositional calculus: axiomatically, through natural deduction, sequent calculi, semantic tableaux. An axiomatic presentation sometimes will start with a list of axioms, the rule of substitution, and the rule of conditional elimination (Co for "conditional out"). For example, one could start with the axiom set {CCpqCCqrCpr, CCNppp, CpCNpq} A1 or {CqCpq, CCpCqrCCpqCpr, CCNpNqCqp} A2. But, let's say we start another way.

Suppose we have the rules of conditional proof alon with substitution and conditional elimination. In other words, we have a rule which allows to introduce any propositional form as a hypothesis, so long as we keep track of its scope, and we have a rule of conditional introduction (Ci for "conditional in"). Suppose also, we don't want to use any axioms for propositional calculus. What sorts of rules of inference do we need?

Well, since the above axiom sets work as sufficient for propositional calculus, we could use the deduction metatheorem from CCNppp to obtain the rule of inference CNpp|-p. So, instead of using A1 we could use R3 {(Cpq, Cqr, p |-r), (CNpp|-p), (p, Np|-q)} along with just conditional elimination and substitution for propositional calculus. But, really, given the rules of conditional proof, we don't need all those rules as primitive rules. (Cpq, Cqr, p |-r) can get derived using just conditional proof as follows:

1 Cpq hypothesis
2 Cqr hypothesis
3 p hypothesis
4 q 3, 1 Co
5 r 4, 2 Co

We can also derive CqCpq, and CCpCqrCCpqCpr and using just conditional proof as follows:

1 | q hypothesis
2 || p hypothesis
3 ||| q hypothesis
4 || Cqq 3-3 Ci
5 || q 1, 4 Co
6 | Cpq 2-5 Ci
7 CqCpq 1-6 Ci

1 | CpCqr hypothesis
2 || Cpq hypothesis
3 ||| p hypothesis
4 ||| q 3, 2 Co
5 ||| Cqr 3, 1 Co
6 ||| r 4, 5 Co
7 || Cpr 3-6 Ci
8 | CCpqCpr 2-7 Ci
9 CCpCqrCCpqCpr 1-8 Ci

So, by the first proof instead of considering A1 with substitution and conditional elimination, we can consider {CCNppp, CpCNpq} N1 along with the rules of conditional proof, conditional elimination and substitution. By the second and third proofs instead of considering A2 we can consider N2 {CCNpNqCqp} along with the same rules which go along with N1. Now using the deduction metatheorem instead of considering N1 we can consider R1 {(CNpp|-p) R1a, (p, Np|-q) R1b} and R2 {(CNpNq, q |-p) MT}. A few weeks ago I took an old logic book and tried to prove certain theorems in both R1 and R2.

So, let's compare some proofs of Peirce's law CCCpqpp in R2 and R1 (of course more proofs exist, the following just give examples). In R1 a proof might go:

1 | CCpqp hypothesis
2 || Np hypothesis
3 ||| p hypothesis
4 ||| q 2, 3 R1b
5 || Cpq 3-4 Ci
6 || p 5, 1 Co
7 | CNpp 2-6 Ci
8 | p R1a
9 CCCpqpp 1-8 Ci

In R2 a proof might go:

1 | CCpqp hypothesis
2 || Np hypothesis
3 ||| p hypothesis
4 |||| Nq hypothesis
5 ||||| Np hypothesis
6 |||| CNpNp 5-5 Ci
7 |||| Np 2, 6 Co
8 ||| CNqNp 4-7 Ci
9 ||| q 3, 8 MT
10 || Cpq 2-9 Ci
11 || p 10, 1, Co
12 | CNpp 2-11 Ci
13 || Np hypothsis
14 ||| NNCNpp hypothesis
15 |||| NNNp hypothesis
16 ||||| Np hypothesis
17 |||| CNpNp 16-16 Ci
18 |||| Np 13, 17 Co
19 ||| CNNNpNp 15-18 Ci
20 ||| p 13, 12 Co
21 ||| NNp 20, 19 MT
22 || CNNCNppNNp 14-21 Ci
23 || NCNpp 13, 22 MT
24 | CNpNCNpp 13-23 Ci
25 | p 12, 24 MT
26 CCCpqpp 1-25 Ci

Let's also compare proofs of CNNpp and CpNNp. In R1 a proof for CNNpp might go:

1 | NNp hypothesis
2 || Np hypothesis
3 || p 1, 2 R1b
4 | CNpp 2-3 Ci
5 | p 4, R1a
6 CNNpp 1-5 Ci

In R2 a proof for CNNpp might go:

1 | NNp hypothesis
2 || Np hypothesis
3 ||| NNNNp hypothesis
4 |||| NNp hypothesis
5 ||| CNNpNNp 4-4 Ci
6 ||| NNp 1, 5 Co
7 || CNNNNpNNp 3-6 Ci
8 || NNNp 2, 7 MT
9 | CNpNNNp 2-8 Ci
10 | p 1, 9 MT
11 CNNpp 1-10 Ci.

For CpNNp in R1 we can have:

1 | p hypothesis
2 || Np hyopthesis
3 || NNNp 1, 2 R1b
4 | CNpNNNp 2-3 Ci
5 | NNp 4, R2b
6 CpNNp 1-5 Ci

For CpNNp in R2 we can have:

1 | p hypothesis
2 || NNNp hypothesis
3 ||| NNp hypothesis
4 |||| NNNNNp hypothesis
5 ||||| NNNp hypothesis
6 |||| CNNNpNNNp 5-5 Ci
7 |||| NNNp 2, 6 Co
8 ||| CNNNNNpNNNp 4-7 Ci
9 ||| NNNNp 3, 8 MT
10 || CNNpNNNNp 3-9 Ci
11 || Np 2, 10 MT
12 | CNNNpNp 2-11 Ci
13 | NNp 1, 12 MT
14 CpNNp 1-13 Ci.

Expressing All Statement Forms of Two Logical Variables as Conditionals

Every statement form which has only two logical variables can get expressed (i. e. comes as logically equivalent to) with {C, N} as the only set allowed for logical operations. For example, the statement form for conjunction Kxy, can get expressed as NCxNqy. Allowing only C and N as the only logical operations in a formula, can we express all statement forms with only two logical variables as conditionals? In other words, can we express all statement forms with only two logical variables such that C is the primary connective, or equivalently such that in Polish notation C is the first symbol of the statement form?

I answer yes. To see that this holds, you can consider the Sheffer stroke D:D00=1, D01=1, D10=1, D11=0. Since Nx==Dxx, and Dxy==CxNy (in other words we can define N and D in this way, and Nx comes as logically equivalent to Dxx, and Dxy comes as logically equivalent to CxNy), for any statement form which comes as a negation, we can transform it into a conditional, by first transforming it into a statement form with the Sheffer stroke D as its primary connective, and then transform the obtained statement form directly into a conditional.

More concretely one can see that you can do such as follows: let abcd indicate that the statement form has value "a" for the pair (0, 0) where 0 indicates falsity, and 1 truth, "b" for the pair (0, 1), c for the pair (1, 0), and d for the pair (1, 1). So, for example 0101 indicates that the statement form has truth value of false (0) when both arguments are false, has truth value of truth (1) when the first argument is false and the second true, has truth value of false (0) when the first argument is true and the second false, and has truth value of true (1) when both arguments are true. Thus, to show that all statement forms of two variables can get expressed as a conditional we only need to come with one example for 0000 through 1111.

For 0000, CCppNCpp works.
For 0001, from Kpq one can infer NCpNq, from which one can infer DCpNqCpNq, from which it follows that CCpNqNCpNq.
For 0010, from KpNq one can infer NCpq, to DCpqCpq to CCpqNCpq.
For 0011, from p we can infer CCqqp (in general C1y==y).
For 0100, from KqNp to NCqp, DCqpCqp, CCqpNCqp.
For 0101, from q to CCppq.
For 0110, from NEpq to NKCpqCqp to NNCCpqNCqp to CCpqNCqp
For 0111, Apq to CNpq
For 1000, KNpNq to NCNpNNq to NCNpq to DCNpqCNpq to CCNpqNCNpq
For 1001, from Epq to KCpqCqp to NCCpqNCqp to DCCpqNCqpCCpqNCqp to CCCpqNCqpNCCpqNCqp
For 1010, from Nq to CCppNq
For 1011, from NKNpq to NNCNpNq to CNpNq or Cqp
For 1100, from Np to CCqqNp
For 1101, Cpq
For 1110, NKpq to NNCpNq to CpNq
For 1111, Cpp.