3.1 Introduction

The primitive formal system is intended to be as simple as possible, so that we may have confidence in its consistency, and in the correctness of its implementation without either depending on proof in a less well founded formal system, or on proof within itself.

The opacity of the syntax and the inefficiency of the proof rules is acceptable at this level, both of these problems can be addressed without logical extension.

The key characteristics required at this stage are simplicity, consistency, expressiveness (with reference to what can be expressed,, not how it is expressed), and completeness.

3.2 Syntax

atom ::= "S" | "K"

term ::= atom | "(" term term ")"

a,b,c...        are metavariables ranging over atoms.
x,y,t,u,v,w ... are metavariables ranging over terms.
3.3 Axioms
|- K
The "standard interpretations of the terms "S" and "K" are the individuals S and K. and the juxtaposition of two terms denotes the application of the denotation of the one to the other. The algebra of terms is therefore isomorphic to our domain of interpretation. The axiom "|-K" indicates that "K" is our version of the proposition "True".

Theorems of the form (u 't') may be interpreted as assertions that the term t satisfies the predicate represented by the term u.

3.4 Inference Rules

We first define the postfix substitution operator [t/a]:

     a[t/a] = t
     b[t/a] = b (provided b!=a)
     (u v)[t/a] = (u[t/a] v[t/a])
our inference rules are then:
(K) t[u/a]               |- t[((K u)v)/a]
(S) t[((u w)(v w))/a]    |- t[(((S u)v)w)/a]
These rules are the inverse of the reduction relationship which determines the truth of a proposition. They therefore make theorems of just those terms whose denotation is reducible to K. and are therefore sound and complete. Since neither of the inference rules permits the derivation of an atomic theorem, "S" is not a theorem and the system is consistent in the sense of Post.

3.5 Abstraction

In the following sections for illustrative purposes we make liberal use of informal syntactic abbreviations. These are not a part of our primitive formal system, but we expect in due course to deal with such matters in fully formal ways.

The techniques we use for abbreviation fall into two main categories. Firstly we allow arbitrary sequences of letters to be used as the names of terms in our primitive system, Such names are introduced by the notation:

m == U
Where m (or ml,m2...) ranges over names which have not previously been used on the left of a definition, and u is a term of our primitive system expressed either directly or using our conventions for abbreviation. A name thus defined may then be used in all those places where the term on the right hand side of its def inition would otherwise have been permitted,

Our metavariables a,b... and t,u... will henceforth range over atoms and terms, and over abbreviations of atoms and terms. m,ml,m2... will be used to range over previously undefined names, and n,nl,n2... over arbitrary names.

The second technique is the use of suggestive informally defined alternative syntax for important constructs, notably for abstraction and for recursive function definitions.

Other abbreviations include, dropping brackets (taking application as left associative), and using infix notation for some dyadic operations.

The identity function may be defined in terms of S and K:

I   ==          ((S K) K)
We introduce a new construct [n]t, This construct may be thought of as a lambda abstraction term, but is in fact an abbreviation for a combinator extensionally equivalent to the lambda term. Within the term t, the name n loses any value for which it has previously been defined, and is eliminated by the following recursive definition which determines a combinator lacking occurrences of n.

We define [n]t inductively as follows:

[n]n            ==    I

[n]b            ==    (K b)
provided n!=b
[n](u v)        ==    ((S [n]u) [n]v)

[nl,n2]t        ==    [n1][n2]t
and in general
[nl,n2,...nn]t  ==    [nl][n2]...[nn]t
In definitions we may write:
m n == t
m == [n]t
m nl n2 ...  nn == t
m == [nl,n2,... nn] t
3.6 Definitions for encodings

In this section we define an encoding of terms into normal terms, We use the notation 't' for the encoding of a term t.

We define True as the (curried) function which selects the first of two arguments.

True           ==     K
And False as the function which selects its second argument:
False X Y      ==     Y
we may now adopt a sugared syntax for conditional constructs:
If X Then Y Else Z == X Y Z
and use this to define a selection of truth functions:
X And Y       ==       If X Then Y    Else False
X Or Y        ==       If X Then True Else Y
X => y        ==       If X Then Y    Else True

X %lt;=> y       ==       (X => Y) And (Y => X)
Not X         ==       If X Then False Else True
A pair constructor may be defined as function which takes any two values X and Y, and returns a function which X or Y depending on whether it is applied to True or False.
<X,Y> Z       ==       If Z Then X Else Y
The definition of the projection functions is then:
Fst X         ==       X True
Snd X         ==       X False
We may now define our encoding of terms into terms. This is done inductively. The base step is the definition of the encoding for the atomic terms, the induction step is the definition of the encoding for an application in terms of the encodings of the constituent terms.

The base step is then:

'K'           ==      <True,True>

'S'           ==      <True,False>
And the induction step:
Mk_app X Y    ==      <False,<X,Y>>
Mk_app (make application) makes the encoding of an application given the encodings of the constituent terms, We also adopt the special syntactic form:
'XY'          ==       Mk_app 'X' 'Y'
Functions may then be defined for analysing encoded terms, e,g,:
Is_app X      ==       Not (Fst X)
(Is_app tests whether X is the encoding of an application)
Fun X         ==       Fst (Snd X)
(Fun extracts the function part of an encoded application)
Arg X         ==       Snd (Snd X)
(Arg extracts the argument part of an encoded application)

An encoding is itself a term and may therefore be itself (re-)encoded, This allows double encodings such as "K" (='<True,True>'), Note that the encoding of terms cannot be expessed as a term (due to the Church- Rosser property), but its restriction to the encoding of encodings can be expressed as a term (since all encodings are distinct normal terms which can be analysed using Is_app etc.).

3.7 Recursion

Now, in order to permit recursive definitions we introduce the fixed point operator.

Sap F == F F

Y F == Sap [X]F(XX)
Note that Y F = [X]F(XX) ([X]F(XX)) = F ([X]F(XX) [X]F(XX)) = F (Y F) i.e. Y F is a fixed point of F.

Henceforth we will admit recursive definitions, writing:

m == t
instead of
m == Y [m]t
m nl n2 ... nn == t
m = Y [m,nl,n2 .... nn] t

3.8 Decoding and partial encoding

The decoding of encoded terms may now be defined as an example of a term representing a predicate over terms:

Decode X ==     if   Is_app X
                Then (Decode (Fun X)) (Decode (Arg X))
                Else If Snd X
                     Then K
                     Else S
A partial encoding algorithm, defined over the set {T,F} may be defined:
Encode X = If X Then 'T' Else 'F'
Using the full encoding a every recursively enumerable set of terms RET may be represented by some term REP_RET such that:
 for any term t,       |-REP_RET 't'
                           iff t mem RET.
3.9 Remarks on the primitive formalism

The key characteristics idehtif ied in section 3.1 were simplicity, consistency, expressiveness and completeness.

The system is evidently simple.

Its consistency in the sense of Post is immediately evident.

For each recursively enumerable subset of our intended domain of interpretation there is a term which represents that set. For each individual and recursively enumerable set of individuals there is a term which represents the proposition that the individual is a member of the set.

In any formal logic, the ground terms which can be proven to satisfy any given predicate defined in that logic, are recursively enumerable. There is therefore a formal sense in which our logic is as expressive as any formal logic can be. For any arbitrary formal system (assuming a reasonable definition of "formal system") the property of formulae known as theoremhood is expressible in our primitive logic. We therefore believe that the formalism is sufficiently expressive for our purposes, and constitutes a foundation system on which sufficiently rich theories can be constructed by the use of definitions only.

Finally, we can say that the system is complete in the following sense: all the propositions expressible are provable iff true. We cannot express classical negation in the system, i.e. there is no term which denotes the set of unprovable terms. This reflects the fact that the complement of a recursively enumerable set is not in general recursively enumerable.

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