Propositional Equation Reasoning Systems(information, essay, original, Jon Awbrey)
Note. The use of the pronoun we in the reading to follow is intended to reflect the narrative point of view of the participatory observer.
This article develops elementary facts about the formal calculi that we describe as
propositional equation reasoning systems (
PERS). This work follows up on the
alpha graphs that
Charles Sanders Peirce devised as a graphical syntax for
propositional calculus and also on the
calculus of indications that
George Spencer Brown presented in his
Laws of Form.
Formal development
The first order of business is to give the exact forms of the axioms that we use, devolving from Peirce's "
Logical Graphs" via Spencer-Brown's
Laws of Form (LOF). In formal proofs, we use a variation of the annotation scheme from LOF to mark each step of the proof according to which axiom, or
initial, is being invoked to justify the corresponding step of syntactic transformation, whether it applies to graphs or to strings.
Axioms
The axioms are just four in number, and they come in a couple of flavors:
- The arithmetic initials, I1 and I2.
- The algebraic initials, J1 and J2.
o-----------------------------------------------------------o
|
|
| ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ---> Condense ` ` ` ` ` ` ` |
o-----------------------------------------------------------o
o-----------------------------------------------------------o
| |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ---> Refold ` ` ` ` ` ` ` ` |
o-----------------------------------------------------------o
o-----------------------------------------------------------o
| |
|
| ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ---> Delete ` ` ` ` ` ` ` ` |
o-----------------------------------------------------------o
o-----------------------------------------------------------o
| |
|
|
|
|
| ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ---> Collect` ` ` ` ` ` ` ` |
o-----------------------------------------------------------o
|
Notice that all of the axioms in this set have the form of equations. This means that all of the inference steps licensed by them are fully reversible. In the proof annotation scheme used below, a double bar "=====" is used to mark this fact, but it may at times be left to the reader to decide which direction of axiom application is the one that is called for in a particular case.
Peirce introduced these formal equations at a level of abstraction that is one step higher than their customary interpretations as propositional calculi, which two readings he called the Entitative and the Existential interpretations, here referred to as En and Ex, respectively. The early CSP, as in his essay on "Qualitative Logic", and also GSB, emphasized the En interpretation, while the later CSP developed mostly the Ex interpretation. When it comes down to this very primitive level of formal structure, it is important to note the significance of the circumstance that this formal system is a very abstract calculus (VAC), devoid of meaning in the usual logical sense.
Frequently used theorems
C1. Double negation theorem
The first theorem is known as the Double Negation Theorem (DNT).
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` a ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ----> Reflect ` ` ` ` ` ` ` |
o-----------------------------------------------------------o
|
The proof that follows it is derived from the one that was given by George Spencer Brown in his book Laws of Form and credited to two of his students, John Dawes and D.A. Utting. This result is annotated as Consequence 1 (C1) or as Reflection in LOF.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
|
|
|
|
|
| |
o============================= | < I2. Unfold "(())" >=========o
|
|
|
|
|
|
|
|
| |
o============================= | < J1. Insert "(a)" >==========o
|
|
|
|
|
|
|
|
|
|
|
| |
o============================= | < J2. Distribute "((a))" >====o
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
o============================= | < J1. Delete "(a)" >==========o
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
o============================= | < J1. Insert "a" >============o
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
o============================= | < J2. Collect "a" >===========o
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| |
o============================= | < J1. Delete "((a))" >========o
|
|
|
|
|
|
|
|
| |
o============================= | < I2. Refold "(())" >=========o
|
|
|
| |
o============================= | < QED >=======================o
C2. Generation theorem
One theorem of frequent use is the so-called Weed and Seed Theorem. The proof is just an inductive exercise, so we can let it go till later, but it says that a label can be freely distributed or freely erased (retracted or withdrawn) anywhere in a subtree whose root is labeled with that label. The second theorem on the list to be shown here amounts to the inductive base case for the Weed and Seed theorem. It has the LOF name of Generation.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ----> Regenerate` ` ` ` ` ` |
o-----------------------------------------------------------o
|
Here is a proof of the Generation Theorem.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C1. Reflect "a(b)" >========o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < I2. Unfold "(())" >=========o
|
|
| `/` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| /` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < J1. Insert "a" >============o
|
|
| `/` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| /` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < J2. Collect "a" >===========o
|
|
| `/` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| /` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C1. Reflect "a", "b" >======o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < QED >=======================o
C3. Dominant form theorem
The third theorem to be proved here is one that GSB annotates as Integration, but it may also be regarded as a matter of Dominance or Recession among forms.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
| ----> Recess ` ` ` ` ` ` ` `|
o-----------------------------------------------------------o
|
Here is a proof of the Dominant Form Theorem.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C2. Regenerate "a" >========o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < J1. Delete "a" >============o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < QED >=======================o
If you scan the elementary steps that lead up to this point, you will notice two distinct qualities of the proofs so far:
- One brand of proof has that falling off a log and rolling downhill sort of quality that is earnestly to be wished for but seldom to be seen, at least, never so often as we'd wish.
- The other kind, more kith o' death than kind, has a quality strained past mercy, with a how in the heck did anybody ever think of that? sort of subtlety that all too unfortunately rules the roost whenever we begin to extend our practice to more and more compelling theories.
This is, to me, at least, a surprising observation, and though I have no grand conclusion to draw from it at the moment, it occurs to me that it might be a useful measure to keep in mind as we essay forth.
Exemplary proofs
With the meagre means afforded by the axioms and theorems given so far, it is already possible to prove a multitude of much more complex theorems. A couple of all-time favorites are given next.
Peirce's law
(Peirce's law|Main article: Peirce's law)
This section presents a proof of Peirce's law, commonly written:
* [[p ⇒ q] ⇒ p] ⇒ p
The first order of business is present the statement as it appears in the so-called existential interpretation of Peirce's own logical graphs. Here is the statement of Peirce's law, as rendered under the existential interpretation into (the topological dual forms of) Peirce's logical graphs:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
|
Finally, here's the promised proof of Peirce's law:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o================================== | < Collect >==============o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o================================== | < Recess >===============o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o================================== | < Refold >===============o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o================================== | < Delete >===============o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o================================== | < Refold >===============o
|
|
| |
o================================== | < QED >==================o
Praeclarum theorema
Now to take up a more interesting example, here is the statement and a proof of the Praeclarum Theorema or Splendid Theorem of Leibniz.
If a is b and d is c, then ad will be bc.
This is a fine theorem, which is proved in this way:
a is b, therefore ad is bd (by what precedes),
d is c, therefore bd is bc (again by what precedes),
ad is bd, and bd is bc, therefore ad is bc. Q.E.D.
(Leibniz]], Logical Papers, p. 41).
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` | ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o-----------------------------------------------------------o
| |
|
| |
o-----------------------------------------------------------o
|
And here's a neat proof of this nice theorem:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
| ` | ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C1. Reflect "ad(bc)" >======o
|
|
| ` |
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < Weed "a", "d" >=============o
|
|
| ` |
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C1. Reflect "b", "c" >======o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < Weed "bc" >=================o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < C3. Recess "abcd" >=========o
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
| |
o============================= | < I2. Refold "(())" >=========o
|
|
| |
o============================= | < QED >=======================o
Formal extension: Cactus calculus
Let us now extend the CSP-GSB calculus in the following way:
The first extension is the reflective extension of logical graphs, or what may be described as the cactus language, after its principal graph-theoretic data structure. It is generated by generalizing the negation operator "(—)" in a particular direction, treating "(—)" as the controlled, moderated, or reflective negation operator of order 1, and adding another such operator for each integer parameter greater than 1. In sum, these operators are symbolized by bracketed argument lists of the following shapes: "(—)", "(—, —)", "(—, —, —)", and so on, where the number of slots is the order of the reflective negation operator in question.
The formal rule of evaluation for a "k-lobe" or k-operator is:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
|
|
|
|
|
|
|
|
| |
| |
| ` ` ` ` ` >
| |
| |
|
| |
|
| ` `=` `( )` ` |
|
| |
o-----------------------------------------------------------o
|
The interpretation of these operators, read as assertions about the values of their listed arguments, is as follows:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
|
|
|
|
|
|
|
|
| |
| |
|
|
|
|
|
|
| |
|
| |
|
| |
|
| |
|
| |
o-----------------------------------------------------------o
|
Case analysis-synthesis theorem
The task at hand is to lay out what I think of as the pontoon bridge between the model-theoretic and the proof-theoretic shores, and thus between their diverging perspectives on logical procedure, even if I can construct it at a point but so close to their common source that it may not seem like it's worth the candle.
The substance of this principle was known to Boole in the 1850's, tantamount to what we now call the boolean expansion of a propositional expression. The only novelty here resides in a certain manner of presentation, in which we will prove the basic principle from the axioms given before. One name for this rule is the Case Analysis-Synthesis Theorem (CAST).
I am going to revert to my customarily sloppy workshop manners and refer to propositions and proposition expressions on rough analogy with functions and function expressions, which implies that a proposition will be regarded as the chief formal object of discussion, enjoying many proposition expressions, formulas, or sentences that express it, but worst of all I will probably just go ahead and use any and all of these terms as loosely as I see fit, taking a bit of extra care only when I see the need.
Let Q be a proposition with an unspecified, but context-appropriate number of variables, say, none, or x, or x1, … xk, as the case may be. (To be more precise, I should have said "sentence Q".)
- Strings and graphs sans labels are called bare.
- A bare terminal node, "o", is known as a stone.
- A bare terminal edge, "|", is known as a stick.
Let the replacement expression of the form Q[o/x] denote the proposition that results from Q by replacing every token of the variable x with a blank, that is to say, by erasing x.
Let the replacement expression of the form Q[|/x] denote the proposition that results from Q by replacing every token of the variable x with a stick stemming from the site of x.
In the case of a proposition Q, that is, an expression of it, not having a token of the designated variable x, let it be stipulated that Q[o/x] = Q = Q[|/x].
I think that I am at long last ready to state the following:
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
| ` ` ` ` ` ` |
| /x] ` ` |
|
|
| |
o-----------------------------------------------------------o
| |
| /x] (x) ) |
| |
o-----------------------------------------------------------o
|
In order to think of tackling even the roughest sketch toward a proof of this theorem, we need to add a number of axioms and axiom schemata. Because I abandoned proof-theoretic purity somewhere in the middle of grinding this calculus into computational form, I never got around to finding the most elegant and minimal, or anything near a complete set of axioms for the cactus language, so what I list here are just the slimmest rudiments of the hodge-podge of rules of thumb that I have found over time to be necessary and useful in most working settings. Some of these special precepts are probably provable from genuine axioms, but I have yet to go looking for a more proper formulation.
o-----------------------------------------------------------o
|
o-----------------------------------------------------------o
|
|
|
|
| ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` ` |
|
|
|
|