Differential Propositional Calculus • 8

Differential Extensions

An initial universe of discourse A^\bullet supplies the groundwork for any number of further extensions, beginning with the first order differential extension \mathrm{E}A^\bullet.  The construction of \mathrm{E}A^\bullet can be described in the following stages.

  • The initial alphabet \mathfrak{A} = \{ ``a_1", \ldots, ``a_n" \} is extended by a first order differential alphabet \mathrm{d}\mathfrak{A} = \{ ``\mathrm{d}a_1", \ldots, ``\mathrm{d}a_n" \} resulting in a first order extended alphabet \mathrm{E}\mathfrak{A} defined as follows.

    \mathrm{E}\mathfrak{A} ~=~ \mathfrak{A} ~\cup~ \mathrm{d}\mathfrak{A} ~=~ \{ ``a_1", \ldots, ``a_n", ``\mathrm{d}a_1", \ldots, ``\mathrm{d}a_n" \}.

  • The initial basis \mathcal{A} = \{ a_1, \ldots, a_n \} is extended by a first order differential basis \mathrm{d}\mathcal{A} = \{ \mathrm{d}a_1, \ldots, \mathrm{d}a_n \} resulting in a first order extended basis \mathrm{E}\mathcal{A} defined as follows.

    \mathrm{E}\mathcal{A} ~=~ \mathcal{A} ~\cup~ \mathrm{d}\mathcal{A} ~=~ \{ a_1, \ldots, a_n, \mathrm{d}a_1, \ldots, \mathrm{d}a_n \}.

  • The initial space A = \langle a_1, \ldots, a_n \rangle is extended by a first order differential space or tangent space \mathrm{d}A = \langle \mathrm{d}a_1, \ldots, \mathrm{d}a_n \rangle at each point of A, resulting in a first order extended space or tangent bundle space \mathrm{E}A defined as follows.

    \mathrm{E}A ~=~ A ~\times~ \mathrm{d}A ~=~ \langle \mathrm{E}\mathcal{A} \rangle ~=~ \langle \mathcal{A} \cup \mathrm{d}\mathcal{A} \rangle ~=~ \langle a_1, \ldots, a_n, \mathrm{d}a_1, \ldots, \mathrm{d}a_n \rangle.

  • Finally, the initial universe A^\bullet = [ a_1, \ldots, a_n ] is extended by a first order differential universe or tangent universe \mathrm{d}A^\bullet = [ \mathrm{d}a_1, \ldots, \mathrm{d}a_n ] at each point of A^\bullet, resulting in a first order extended universe or tangent bundle universe \mathrm{E}A^\bullet defined as follows.

    \mathrm{E}A^\bullet ~=~ [ \mathrm{E}\mathcal{A} ] ~=~ [ \mathcal{A} ~\cup~ \mathrm{d}\mathcal{A} ] ~=~ [ a_1, \ldots, a_n, \mathrm{d}a_1, \ldots, \mathrm{d}a_n ].

    This gives \mathrm{E}A^\bullet a type defined as follows.

    [ \mathbb{B}^n \times \mathbb{D}^n ] ~=~ (\mathbb{B}^n \times \mathbb{D}^n\ +\!\!\to \mathbb{B}) ~=~ (\mathbb{B}^n \times \mathbb{D}^n, \mathbb{B}^n \times \mathbb{D}^n \to \mathbb{B}).

A proposition in a differential extension of a universe of discourse is called a differential proposition and forms the analogue of a system of differential equations in ordinary calculus.  With these constructions, the first order extended universe \mathrm{E}A^\bullet and the first order differential propositions f : \mathrm{E}A \to \mathbb{B}, we arrive at the foothills of differential logic.

Table 11 summarizes the notations needed to describe the first order differential extensions of propositional calculi in a systematic manner.

\text{Table 11. Differential Extension} \stackrel{_\bullet}{} \text{Basic Notation}
Differential Extension • Basic Notation

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Habitations

Our reach exceeds our rut and yet
We grasp but what we drag into it.

Re: Scott AaronsonA Coronavirus Poem

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Differential Propositional Calculus • 7

Special Classes of Propositions (concl.)

Last and literally least in extent, we examine the family of singular propositions in a 3-dimensional universe of discourse.

In our model of propositions as mappings of a universe of discourse to a set of two values, in other words, indicator functions of the form f : X \to \mathbb{B}, singular propositions are those singling out the minimal distinct regions of the universe, represented by single cells of the corresponding venn diagram.

Singular Propositions

Singular Propositions May Be Written As Products

In a universe of discourse based on three boolean variables, p, q, r, there are 2^3 = 8 singular propositions.  Their venn diagrams are shown in Figure 10.

Singular Propositions on Three Variables

\text{Figure 10.} ~~ \text{Singular Propositions} : \mathbb{B}^3 \to \mathbb{B}

At the top is the venn diagram for the singular proposition of rank 3, corresponding to the boolean product pqr and identical with the positive proposition of rank 3.

Next are the venn diagrams for the three singular propositions of rank 2, which may be expressed by the following three forms, respectively:

pr \texttt{(} q \texttt{)}, \qquad  qr \texttt{(} p \texttt{)}, \qquad  pq \texttt{(} r \texttt{)}.

Next are the three singular propositions of rank 1, which may be expressed by the following three forms, respectively:

q \texttt{(} p \texttt{)(} r \texttt{)}, \qquad  p \texttt{(} q \texttt{)(} r \texttt{)}, \qquad  r \texttt{(} p \texttt{)(} q \texttt{)}.

At the bottom is the singular proposition of rank 0, which may be expressed by the following form:

\texttt{(} p \texttt{)(} q \texttt{)(} r \texttt{)}.

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Differential Propositional Calculus • 6

Special Classes of Propositions (cont.)

Next we take up the family of positive propositions and follow the same plan as before, tracing the rule of their formation in the case of a 3-dimensional universe of discourse.

Positive Propositions

Positive Propositions May Be Written As Products

In a universe of discourse based on three boolean variables, p, q, r, there are 2^3 = 8 positive propositions, taking the shapes shown in Figure 9.

Positive Propositions on Three Variables

\text{Figure 9.} ~~ \text{Positive Propositions} : \mathbb{B}^3 \to \mathbb{B}

At the top is the venn diagram for the positive proposition of rank 3, corresponding to the boolean product or logical conjunction pqr.

Next are the venn diagrams for the three positive propositions of rank 2, corresponding to the three boolean products, pr, qr, pq, respectively.

Next are the three positive propositions of rank 1, which are none other than the three basic propositions, p, q, r.

At the bottom is the positive proposition of rank 0, the everywhere true proposition or the constant 1 function, which may be expressed by the form \texttt{((}~\texttt{))} or by a simple 1.

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Differential Propositional Calculus • 5

Special Classes of Propositions (cont.)

Let’s pause at this point and get a better sense of how our special classes of propositions are structured and how they relate to propositions in general.  We can do this by recruiting our visual imaginations and drawing up a sufficient budget of venn diagrams for each family of propositions.  The case for 3 variables is exemplary enough for a start.

Linear Propositions

Linear Propositions May Be Written As Sums

One thing to keep in mind about these sums is that the values in \mathbb{B} = \{ 0, 1 \} are added “modulo 2”, that is, in such a way that 1 + 1 = 0.

In a universe of discourse based on three boolean variables, p, q, r, the linear propositions take the shapes shown in Figure 8.

Linear Propositions on Three Variables

\text{Figure 8.} ~~ \text{Linear Propositions} : \mathbb{B}^3 \to \mathbb{B}

At the top is the venn diagram for the linear proposition of rank 3, which may be expressed by any one of the following three forms:

\texttt{(} p \texttt{,(} q \texttt{,} r \texttt{))}, \qquad  \texttt{((} p \texttt{,} q \texttt{),} r \texttt{)}, \qquad  p + q + r.

Next are the venn diagrams for the three linear propositions of rank 2, which may be expressed by the following three forms, respectively:

\texttt{(} p \texttt{,} r \texttt{)}, \qquad  \texttt{(} q \texttt{,} r \texttt{)}, \qquad  \texttt{(} p \texttt{,} q \texttt{)}.

Next are the three linear propositions of rank 1, which are none other than the three basic propositions, p, q, r.

At the bottom is the linear proposition of rank 0, the everywhere false proposition or the constant 0 function, which may be expressed by the form \texttt{(} ~ \texttt{)} or by a simple 0.

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Differential Propositional Calculus • 4

Special Classes of Propositions

Before moving on, let’s unpack some of the assumptions, conventions, and implications involved in the array of concepts and notations introduced above.

A universe of discourse A^\bullet = [a_1, \ldots, a_n] qualified by the logical features a_1, \ldots, a_n is a set A plus the set of all functions from the space A to the boolean domain \mathbb{B} = \{ 0, 1 \}.  There are 2^n elements in A, often pictured as the cells of a venn diagram or the nodes of a hypercube.  There are 2^{2^n} possible functions from A to \mathbb{B}, accordingly pictured as all the ways of painting the cells of a venn diagram or the nodes of a hypercube with a palette of two colors.

A logical proposition about the elements of A is either true or false of each element in A, while a function f : A \to \mathbb{B} evaluates to 1 or 0 on each element of A.  The analogy between logical propositions and boolean-valued functions is close enough to adopt the latter as models of the former and simply refer to the functions f : A \to \mathbb{B} as propositions about the elements of A.

The full set of propositions f : A \to \mathbb{B} contains a number of smaller classes deserving of special attention.

A basic proposition in the universe of discourse [a_1, \ldots, a_n] is one of the propositions in the set \{ a_1, \ldots, a_n \}.  There are of course exactly n of these.  Depending on the context, basic propositions may also be called coordinate propositions or simple propositions.

Among the 2^{2^n} propositions in [a_1, \ldots, a_n] are several families numbering 2^n propositions each which take on special forms with respect to the basis \{ a_1, \ldots, a_n \}.  Three of these families are especially prominent in the present context, the linear, the positive, and the singular propositions.  Each family is naturally parameterized by the coordinate n-tuples in \mathbb{B}^n and falls into n + 1 ranks, with a binomial coefficient \tbinom{n}{k} giving the number of propositions having rank or weight k in their class.

In each case the rank k ranges from 0 to n and counts the number of positive appearances of the coordinate propositions a_1, \ldots, a_n in the resulting expression.  For example, when n = 3 the linear proposition of rank 0 is 0, the positive proposition of rank 0 is 1, and the singular proposition of rank 0 is \texttt{(} a_1 \texttt{)} \texttt{(} a_2 \texttt{)} \texttt{(} a_3 \texttt{)}.

The basic propositions a_i : \mathbb{B}^n \to \mathbb{B} are both linear and positive.  So these two kinds of propositions, the linear and the positive, may be viewed as two different ways of generalizing the class of basic propositions.

Finally, it is important to note that all of the above distinctions are relative to the choice of a particular logical basis \mathcal{A} = \{ a_1, \ldots, a_n \}.  A singular proposition with respect to the basis \mathcal{A} will not remain singular if \mathcal{A} is extended by a number of new and independent features.  Even if one keeps to the original set of pairwise options \{ a_i \} \cup \{ \texttt{(} a_i \texttt{)} \} to pick out a new basis, the sets of linear propositions and positive propositions are both determined by the choice of basic propositions, and this whole determination is tantamount to the purely conventional choice of a cell as origin.

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Differential Propositional Calculus • 3

Formal Development

The preceding discussion outlined the ideas leading to the differential extension of propositional logic.  The next task is to lay out the concepts and terminology needed to describe various orders of differential propositional calculi.

Elementary Notions

Logical description of a universe of discourse begins with a collection of logical signs.  For simplicity in a first approach we assume the signs are collected in the form of a finite alphabet, \mathfrak{A} = \{``a_1", \ldots, ``a_n"\}.  The signs are interpreted as denoting logical features, for example, properties of objects in the universe of discourse or simple propositions about those objects.  Corresponding to the alphabet \mathfrak{A} there is then a set of logical features, \mathcal{A} = \{ a_1, \ldots, a_n \}.

A set of logical features \mathcal{A} = \{ a_1, \ldots, a_n \} affords a basis for generating an n-dimensional universe of discourse, written A^\bullet = [ \mathcal{A} ] = [ a_1, \ldots, a_n ].  It is useful to consider a universe of discourse as a categorical object incorporating both the set of points A = \langle a_1, \ldots, a_n \rangle and the set of propositions A^\uparrow = \{ f : A \to \mathbb{B} \} implicit with the ordinary picture of a venn diagram on n features.  Accordingly, the universe of discourse A^\bullet may be regarded as an ordered pair (A, A^\uparrow) having the type (\mathbb{B}^n, (\mathbb{B}^n \to \mathbb{B})) and this last type designation may be abbreviated as \mathbb{B}^n\ +\!\!\to \mathbb{B}, or even more succinctly as [ \mathbb{B}^n ].  For convenience, the data type of a finite set on n elements may be indicated by either one of the equivalent notations, [n] or \mathbf{n}.

Table 7 summarizes the notations needed to describe ordinary propositional calculi in a systematic fashion.

\text{Table 7. Propositional Calculus} \stackrel{_\bullet}{} \text{Basic Notation}
Propositional Calculus • Basic Notation

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Differential Propositional Calculus • 2

Cactus Calculus

Table 6 outlines a syntax for propositional calculus based on two types of logical connectives, both of variable k-ary scope.

  • A bracketed sequence of propositional expressions \texttt{(} e_1 \texttt{,} e_2 \texttt{,} \ldots \texttt{,} e_{k-1} \texttt{,} e_k \texttt{)} is taken to mean exactly one of the propositions e_1, e_2, \ldots, e_{k-1}, e_k is false, in other words, their minimal negation is true.
  • A concatenated sequence of propositional expressions e_1 ~ e_2 ~ \ldots ~ e_{k-1} ~ e_k is taken to mean every one of the propositions e_1, e_2, \ldots, e_{k-1}, e_k is true, in other words, their logical conjunction is true.

\text{Table 6. Syntax and Semantics of a Calculus for Propositional Logic}
Syntax and Semantics of a Calculus for Propositional Logic

All other propositional connectives can be obtained through combinations of the above two forms.  Strictly speaking, the concatenation form is dispensable in light of the bracket form, but it is convenient to maintain it as an abbreviation for more complicated bracket expressions.  While working with expressions solely in propositional calculus, it is easiest to use plain parentheses for logical connectives.  In contexts where parentheses are needed for other purposes “teletype” parentheses \texttt{(} \ldots \texttt{)} or barred parentheses (\!| \ldots |\!) may be used for logical operators.

The briefest expression for logical truth is the empty word, abstractly denoted \boldsymbol\varepsilon or \boldsymbol\lambda in formal languages, where it forms the identity element for concatenation.  It may be given visible expression in this context by means of the logically equivalent form \texttt{((} ~ \texttt{))}, or, especially if operating in an algebraic context, by a simple 1.  Also when working in an algebraic mode, the plus sign {+} may be used for exclusive disjunction.  For example, we have the following paraphrases of algebraic expressions:

\begin{matrix}  x + y ~=~ \texttt{(} x \texttt{,} y \texttt{)}  \\[6pt]  x + y + z ~=~ \texttt{((} x \texttt{,} y \texttt{),} z \texttt{)} ~=~ \texttt{(} x \texttt{,(} y \texttt{,} z \texttt{))}  \end{matrix}

It is important to note the last expressions are not equivalent to the triple bracket \texttt{(} x \texttt{,} y \texttt{,} z \texttt{)}.

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Differential Propositional Calculus • 1

A differential propositional calculus is a propositional calculus extended by a set of terms for describing aspects of change and difference, for example, processes taking place in a universe of discourse or transformations mapping a source universe to a target universe.

Casual Introduction

Consider the situation represented by the venn diagram in Figure 1.

Figure 1. Local Habitations, And Names
\text{Figure 1. Local Habitations, And Names}

The area of the rectangle represents a universe of discourse, X.  The universe under discussion may be a population of individuals having various additional properties or it may be a collection of locations occupied by various individuals.  The area of the “circle” represents the individuals having the property q or the locations in the corresponding region Q.  Four individuals, a, b, c, d, are singled out by name.  It happens that b and c currently reside in region Q while a and d do not.

Now consider the situation represented by the venn diagram in Figure 2.

Figure 2. Same Names, Different Habitations
\text{Figure 2. Same Names, Different Habitations}

Figure 2 differs from Figure 1 solely in the circumstance that the object c is outside the region Q while the object d is inside the region Q.  So far, nothing says our encountering these Figures in this order is other than purely accidental but if we interpret this sequence of frames as a “moving picture” representation of their natural order in a temporal process then it would be natural to suppose a and b have remained as they were with regard to the quality q while c and d have changed their standings in that respect.  In particular, c has moved from the region where q is true to the region where q is false while d has moved from the region where q is false to the region where q is true.

Figure 3 returns to the situation in Figure 1, but this time interpolates a new quality specifically tailored to account for the relation between Figure 1 and Figure 2.

Figure 3. Back, To The Future
\text{Figure 3. Back, To The Future}

This new quality, \mathrm{d}q, is an example of a differential quality, since its absence or presence qualifies the absence or presence of change occurring in another quality.  As with any other quality, it is represented in the venn diagram by means of a “circle” distinguishing two halves of the universe of discourse, in this case, the portions of X outside and inside the region \mathrm{d}Q.

Figure 1 represents a universe of discourse, X, together with a basis of discussion, \{ q \}, for expressing propositions about the contents of that universe.  Once the quality q is given a name, say, the symbol ``q", we have the basis for a formal language specifically cut out for discussing X in terms of q.  This language is more formally known as the propositional calculus with alphabet \{ ``q" \}.

In the context marked by X and \{ q \} there are just four distinct pieces of information which can be expressed in the corresponding propositional calculus, namely, the constant proposition \text{false}, the negative proposition \lnot q, the positive proposition q, and the constant proposition \text{true}.

For example, referring to the points in Figure 1, the constant proposition \text{false} holds of no points, the negative proposition \lnot q holds of a and d, the positive proposition q holds of b and c, and the constant proposition \text{true} holds of all points in the sample.

Figure 3 extends the basis of description for the space X to a set of two qualities \{q, \mathrm{d}q\} and the corresponding terms of description to an alphabet of two symbols \{``q", ``\mathrm{d}q"\}.

Any propositional calculus over two basic propositions allows for the expression of sixteen propositions all together.  Salient among those propositions in the present setting are the four which single out the individual sample points at the initial moment of observation.  Table 4 lists the initial state descriptions, using overlines to express logical negations.

\text{Table 4. Initial State Descriptions}

Initial State Descriptions

Table 5 shows the rules of inference responsible for giving the differential quality \mathrm{d}q its meaning in practice.

\text{Table 5. Differential Inference Rules}

Differential Inference Rules

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Differential Propositional Calculus • Overview

The most fundamental concept in cybernetics is that of “difference”, either that two things are recognisably different or that one thing has changed with time.

W. Ross Ashby • An Introduction to Cybernetics

Here’s the outline of a sketch I wrote on differential propositional calculi, which extend propositional calculi by adding terms for describing aspects of change and difference, for example, processes taking place in a universe of discourse or transformations mapping a source universe to a target universe.  I wrote this as an intuitive introduction to differential logic, which is my best effort so far at dealing with the ancient and persistent problems of treating diversity and mutability in logical terms.  I’ll be looking at ways to improve this draft as I serialize it to my blog.

Part 1

Casual Introduction

Cactus Calculus

Part 2

Formal_Development

Elementary Notions

Special Classes of Propositions

Linear Propositions

Positive Propositions

Singular Propositions

Differential Extensions

Appendices

Appendices

Appendix 1. Propositional Forms and Differential Expansions

Table A1. Propositional Forms on Two Variables

Table A2. Propositional Forms on Two Variables

Table A3. Ef Expanded Over Differential Features

Table A4. Df Expanded Over Differential Features

Table A5. Ef Expanded Over Ordinary Features

Table A6. Df Expanded Over Ordinary Features

Appendix 2. Differential Forms

Table A7. Differential Forms Expanded on a Logical Basis

Table A8. Differential Forms Expanded on an Algebraic Basis

Table A9. Tangent Proposition as Pointwise Linear Approximation

Table A10. Taylor Series Expansion Df = df + d²f

Table A11. Partial Differentials and Relative Differentials

Table A12. Detail of Calculation for the Difference Map

Appendix 3. Computational Details

Operator Maps for the Logical Conjunction f8(u, v)

Computation of εf8
Computation of Ef8
Computation of Df8
Computation of df8
Computation of rf8
Computation Summary for Conjunction

Operator Maps for the Logical Equality f9(u, v)

Computation of εf9
Computation of Ef9
Computation of Df9
Computation of df9
Computation of rf9
Computation Summary for Equality

Operator Maps for the Logical Implication f11(u, v)

Computation of εf11
Computation of Ef11
Computation of Df11
Computation of df11
Computation of rf11
Computation Summary for Implication

Operator Maps for the Logical Disjunction f14(u, v)

Computation of εf14
Computation of Ef14
Computation of Df14
Computation of df14
Computation of rf14
Computation Summary for Disjunction

Appendix 4. Source Materials

Appendix 5. Various Definitions of the Tangent Vector

References

References

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