Differential Propositional Calculus • 2

Casual Introduction (cont.)

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.

Nothing says our encountering the Figures in the above order is other than purely accidental but if we interpret the 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.

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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 the 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 with the property q or the locations in the corresponding region Q.  Four individuals, a, b, c, d, are singled out by name.  As it happens, b and c currently reside in region Q while a and d do not.

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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

Differential logic is the component of logic whose object is the description of variation — the aspects of change, difference, distribution, and diversity — in universes of discourse subject to logical description.  To the extent a logical inquiry makes use of a formal system, its differential component treats the use of a differential logical calculus — a formal system with the expressive capacity to describe change and diversity in logical universes of discourse.

In accord with the strategy of approaching logical systems in stages, first gaining a foothold in propositional logic and advancing on those grounds, we may set our first stepping stones toward differential logic in differential propositional calculi — propositional calculi extended by sets 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.

What follows is the outline of a sketch on differential propositional calculus intended as an intuitive introduction to the larger subject of differential logic, which amounts in turn to my best effort so far at dealing with the ancient and persistent problems of treating diversity and mutability in logical terms.

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

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Differential Logic • Overview

A reader once told me “venn diagrams are obsolete” and of course we all know how unwieldy they become as our universes of discourse expand beyond four or five dimensions.  Indeed, one of the first lessons I learned when I set about implementing Peirce’s graphs and Spencer Brown’s forms on the computer is that 2‑dimensional representations of logic quickly become death traps on numerous conceptual and computational counts.

Still, venn diagrams do us good service at the outset in visualizing the relationships among extensional, functional, and intensional aspects of logic.  A facility with those connections is critical to the computational applications and statistical generalizations of propositional logic commonly used in mathematical and empirical practice.  All things considered, then, it is useful to make as visible as possible the links between variant styles of imagery in logical representation — and that is what I hoped to do in the sketch of Differential Logic outlined below.

Part 1

Introduction

Cactus Language for Propositional Logic

Differential Expansions of Propositions

Bird’s Eye View

Worm’s Eye View

Panoptic View • Difference Maps

Panoptic View • Enlargement Maps

Part 2

Propositional Forms on Two Variables

Transforms Expanded over Ordinary and Differential Variables

Enlargement Map Expanded over Ordinary Variables

Enlargement Map Expanded over Differential Variables

Difference Map Expanded over Ordinary Variables

Difference Map Expanded over Differential Variables

Operational Representation

Part 3

Field Picture

Differential Fields

Propositions and Tacit Extensions

Enlargement and Difference Maps

Tangent and Remainder Maps

Least Action Operators

Goal-Oriented Systems

Further Reading

Document History

Document History

Differential Logic • Ontology List 2002

Dynamics And Logic • Inquiry List 2004

Dynamics And Logic • NKS Forum 2004

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Differential Logic • 18

Tangent and Remainder Maps

If we follow the classical line which singles out linear functions as ideals of simplicity then we may complete the analytic series of the proposition f = pq : X \to \mathbb{B} in the following way.

The next venn diagram shows the differential proposition \mathrm{d}f = \mathrm{d}(pq) : \mathrm{E}X \to \mathbb{B} we get by extracting the linear approximation to the difference map \mathrm{D}f = \mathrm{D}(pq) : \mathrm{E}X \to \mathbb{B} at each cell or point of the universe X.  What results is the logical analogue of what would ordinarily be called the differential of pq but since the adjective differential is being attached to just about everything in sight the alternative name tangent map is commonly used for \mathrm{d}f whenever it’s necessary to single it out.

Tangent Map d(pq) : EX → B
\text{Tangent Map}~ \mathrm{d}(pq) : \mathrm{E}X \to \mathbb{B}

To be clear about what’s being indicated here, it’s a visual way of summarizing the following data.

\begin{array}{rcccccc}  \mathrm{d}(pq)  & = &  p & \cdot & q & \cdot &  \texttt{(} \mathrm{d}p \texttt{,} \mathrm{d}q \texttt{)}  \\[4pt]  & + &  p & \cdot & \texttt{(} q \texttt{)} & \cdot &  \mathrm{d}q  \\[4pt]  & + &  \texttt{(} p \texttt{)} & \cdot & q & \cdot &  \mathrm{d}p  \\[4pt]  & + &  \texttt{(} p \texttt{)} & \cdot & \texttt{(} q \texttt{)} & \cdot & 0  \end{array}

To understand the extended interpretations, that is, the conjunctions of basic and differential features which are being indicated here, it may help to note the following equivalences.

\begin{matrix}  \texttt{(} \mathrm{d}p \texttt{,} \mathrm{d}q \texttt{)}  & = &  \texttt{~} \mathrm{d}p \texttt{~} \texttt{(} \mathrm{d}q \texttt{)}  & + &  \texttt{(} \mathrm{d}p \texttt{)} \texttt{~} \mathrm{d}q \texttt{~}  \\[4pt]  dp  & = &  \texttt{~} \mathrm{d}p \texttt{~} \texttt{~} \mathrm{d}q \texttt{~}  & + &  \texttt{~} \mathrm{d}p \texttt{~} \texttt{(} \mathrm{d}q \texttt{)}  \\[4pt]  \mathrm{d}q  & = &  \texttt{~} \mathrm{d}p \texttt{~} \texttt{~} \mathrm{d}q \texttt{~}  & + &  \texttt{(} \mathrm{d}p \texttt{)} \texttt{~} \mathrm{d}q \texttt{~}  \end{matrix}

Capping the analysis of the proposition pq in terms of succeeding orders of linear propositions, the final venn diagram of the series shows the remainder map \mathrm{r}(pq) : \mathrm{E}X \to \mathbb{B}, which happens to be linear in pairs of variables.

Remainder r(pq) : EX → B
\text{Remainder}~ \mathrm{r}(pq) : \mathrm{E}X \to \mathbb{B}

Reading the arrows off the map produces the following data.

\begin{array}{rcccccc}  \mathrm{r}(pq)  & = & p & \cdot & q & \cdot & \mathrm{d}p ~ \mathrm{d}q  \\[4pt]  & + & p & \cdot & \texttt{(} q \texttt{)} & \cdot & \mathrm{d}p ~ \mathrm{d}q  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & q & \cdot & \mathrm{d}p ~ \mathrm{d}q  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & \texttt{(} q \texttt{)} & \cdot & \mathrm{d}p ~ \mathrm{d}q  \end{array}

In short, \mathrm{r}(pq) is a constant field, having the value \mathrm{d}p~\mathrm{d}q at each cell.

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Differential Logic • 17

Enlargement and Difference Maps

Continuing with the example pq : X \to \mathbb{B}, the following venn diagram shows the enlargement or shift map \mathrm{E}(pq) : \mathrm{E}X \to \mathbb{B} in the same style of field picture we drew for the tacit extension \boldsymbol\varepsilon (pq) : \mathrm{E}X \to \mathbb{B}.

Enlargement E(pq) : EX → B
\text{Enlargement}~ \mathrm{E}(pq) : \mathrm{E}X \to \mathbb{B}

\begin{array}{rcccccc}  \mathrm{E}(pq)   & = & p & \cdot & q & \cdot &  \texttt{(} \mathrm{d}p \texttt{)(} \mathrm{d}q \texttt{)}  \\[4pt]  & + & p & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{(} \mathrm{d}p \texttt{)~} \mathrm{d}q \texttt{~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & q & \cdot &  \texttt{~} \mathrm{d}p \texttt{~(} \mathrm{d}q \texttt{)}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{~} \mathrm{d}p \texttt{~~} \mathrm{d}q \texttt{~}  \end{array}

A very important conceptual transition has just occurred here, almost tacitly, as it were.  Generally speaking, having a set of mathematical objects of compatible types, in this case the two differential fields \boldsymbol\varepsilon f and \mathrm{E}f, both of the type \mathrm{E}X \to \mathbb{B}, is very useful, because it allows us to consider those fields as integral mathematical objects which can be operated on and combined in the ways we usually associate with algebras.

In the present case one notices the tacit extension \boldsymbol\varepsilon f and the enlargement \mathrm{E}f are in a sense dual to each other.  The tacit extension \boldsymbol\varepsilon f indicates all the arrows out of the region where f is true and the enlargement \mathrm{E}f indicates all the arrows into the region where f is true.  The only arc they have in common is the no‑change loop \texttt{(} \mathrm{d}p \texttt{)(} \mathrm{d}q \texttt{)} at pq.  If we add the two sets of arcs in mod 2 fashion then the loop of multiplicity 2 zeroes out, leaving the 6 arrows of \mathrm{D}(pq) = \boldsymbol\varepsilon(pq) + \mathrm{E}(pq) shown in the following venn diagram.

Differential D(pq) : EX → B
\text{Difference}~ \mathrm{D}(pq) : \mathrm{E}X \to \mathbb{B}

\begin{array}{rcccccc}  \mathrm{D}(pq)   & = & p & \cdot & q & \cdot &  \texttt{((} \mathrm{d}p \texttt{)(} \mathrm{d}q \texttt{))}  \\[4pt]  & + & p & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{~(} \mathrm{d}p \texttt{)~} \mathrm{d}q \texttt{~~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & q & \cdot &  \texttt{~~} \mathrm{d}p \texttt{~(} \mathrm{d}q \texttt{)~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & \texttt{(}q \texttt{)} & \cdot &  \texttt{~~} \mathrm{d}p \texttt{~~} \mathrm{d}q \texttt{~~}  \end{array}

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Differential Logic • 16

Propositions and Tacit Extensions

Now that we’ve introduced the field picture as an aid to visualizing propositions and their analytic series, a pleasing way to picture the relationship of a proposition f : X \to \mathbb{B} to its enlargement or shift map \mathrm{E}f : \mathrm{E}X \to \mathbb{B} and its difference map \mathrm{D}f : \mathrm{E}X \to \mathbb{B} can now be drawn.

To illustrate the possibilities, let’s return to the differential analysis of the conjunctive proposition f(p, q) = pq and give its development a slightly different twist at the appropriate point.

The proposition pq : X \to \mathbb{B} is shown again in the venn diagram below.  In the field picture it may be seen as a scalar field — analogous to a potential hill in physics but in logic amounting to a potential plateau — where the shaded region indicates an elevation of 1 and the unshaded region indicates an elevation of 0.

Proposition pq : X → B
\text{Proposition}~ pq : X \to \mathbb{B}

Given a proposition f : X \to \mathbb{B}, the tacit extension of f to \mathrm{E}X is denoted \boldsymbol\varepsilon f : \mathrm{E}X \to \mathbb{B} and defined by the equation \boldsymbol\varepsilon f = f, so it’s really just the same proposition residing in a bigger universe.  Tacit extensions formalize the intuitive idea that a function on a given set of variables can be extended to a function on a superset of those variables in such a way that the new function obeys the same constraints on the old variables, with a “don’t care” condition on the new variables.

The tacit extension of the scalar field pq : X \to \mathbb{B} to the differential field \boldsymbol\varepsilon (pq) : \mathrm{E}X \to \mathbb{B} is shown in the following venn diagram.

Tacit Extension ε(pq) : EX → B
\text{Tacit Extension}~ \boldsymbol\varepsilon (pq) : \mathrm{E}X \to \mathbb{B}

\begin{array}{rcccccc}  \boldsymbol\varepsilon (pq)  & = &  p & \cdot & q & \cdot &  \texttt{(} \mathrm{d}p \texttt{)}  \texttt{(} \mathrm{d}q \texttt{)}  \\[4pt]  & + &  p & \cdot & q & \cdot &  \texttt{(} \mathrm{d}p \texttt{)}  \texttt{~} \mathrm{d}q \texttt{~}  \\[4pt]  & + &  p & \cdot & q & \cdot &  \texttt{~} \mathrm{d}p \texttt{~}  \texttt{(} \mathrm{d}q \texttt{)}  \\[4pt]  & + &  p & \cdot & q & \cdot &  \texttt{~} \mathrm{d}p \texttt{~}  \texttt{~} \mathrm{d}q \texttt{~}  \end{array}

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Differential Logic • 15

Differential Fields

The structure of a differential field may be described as follows.  With each point of X there is associated an object of the following type:  a proposition about changes in X, that is, a proposition g : \mathrm{d}X \to \mathbb{B}.  In that frame of reference, if {X^\bullet} is the universe generated by the set of coordinate propositions \{ p, q \} then \mathrm{d}X^\bullet is the differential universe generated by the set of differential propositions \{ \mathrm{d}p, \mathrm{d}q \}.  The differential propositions \mathrm{d}p and \mathrm{d}q may thus be interpreted as indicating ``\text{change in}~ p" and ``\text{change in}~ q", respectively.

A differential operator \mathrm{W}, of the first order type we are currently considering, takes a proposition f : X \to \mathbb{B} and gives back a differential proposition \mathrm{W}f : \mathrm{E}X \to \mathbb{B}.  In the field view of the scene, we see the proposition f : X \to \mathbb{B} as a scalar field and we see the differential proposition \mathrm{W}f : \mathrm{E}X \to \mathbb{B} as a vector field, specifically, a field of propositions about contemplated changes in X.

The field of changes produced by \mathrm{E} on pq is shown in the following venn diagram.

Enlargement E(pq) : EX → B
\text{Enlargement}~ \mathrm{E}(pq) : \mathrm{E}X \to \mathbb{B}

\begin{array}{rcccccc}  \mathrm{E}(pq)   & = & p & \cdot & q & \cdot &  \texttt{(} \mathrm{d}p \texttt{)(} \mathrm{d}q \texttt{)}  \\[4pt]  & + & p & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{(} \mathrm{d}p \texttt{)~} \mathrm{d}q \texttt{~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & q & \cdot &  \texttt{~} \mathrm{d}p \texttt{~(} \mathrm{d}q \texttt{)}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{~} \mathrm{d}p \texttt{~~} \mathrm{d}q \texttt{~}  \end{array}

The differential field \mathrm{E}(pq) specifies the changes which need to be made from each point of X in order to reach one of the models of the proposition pq, that is, in order to satisfy the proposition pq.

The field of changes produced by \mathrm{D} on pq is shown in the following venn diagram.

Differential D(pq) : EX → B
\text{Difference}~ \mathrm{D}(pq) : \mathrm{E}X \to \mathbb{B}

\begin{array}{rcccccc}  \mathrm{D}(pq)   & = & p & \cdot & q & \cdot &  \texttt{((} \mathrm{d}p \texttt{)(} \mathrm{d}q \texttt{))}  \\[4pt]  & + & p & \cdot & \texttt{(} q \texttt{)} & \cdot &  \texttt{~(} \mathrm{d}p \texttt{)~} \mathrm{d}q \texttt{~~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & q & \cdot &  \texttt{~~} \mathrm{d}p \texttt{~(} \mathrm{d}q \texttt{)~}  \\[4pt]  & + & \texttt{(} p \texttt{)} & \cdot & \texttt{(}q \texttt{)} & \cdot &  \texttt{~~} \mathrm{d}p \texttt{~~} \mathrm{d}q \texttt{~~}  \end{array}

The differential field \mathrm{D}(pq) specifies the changes which need to be made from each point of X in order to feel a change in the felt value of the field pq.

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Differential Logic • 14

Field Picture

Let us summarize the outlook on differential logic we’ve reached so far.  We’ve been considering a class of operators on universes of discourse, each of which takes us from considering one universe of discourse X^\bullet to considering a larger universe of discourse \mathrm{E}X^\bullet.  An operator \mathrm{W} of that general type, namely, \mathrm{W} : X^\bullet \to \mathrm{E}X^\bullet, acts on each proposition f : X \to \mathbb{B} of the source universe {X^\bullet} to produce a proposition \mathrm{W}f : \mathrm{E}X \to \mathbb{B} of the target universe \mathrm{E}X^\bullet.

The operators we’ve examined so far are the enlargement or shift operator \mathrm{E} : X^\bullet \to \mathrm{E}X^\bullet and the difference operator \mathrm{D} : X^\bullet \to \mathrm{E}X^\bullet.  The operators \mathrm{E} and \mathrm{D} act on propositions in X^\bullet, that is, propositions of the form f : X \to \mathbb{B} which amount to propositions about the subject matter of X, and they produce propositions of the form \mathrm{E}f, \mathrm{D}f : \mathrm{E}X \to \mathbb{B} which amount to propositions about specified collections of changes conceivably occurring in X.

At this point we find ourselves in need of visual representations, suitable arrays of concrete pictures to anchor our more earthy intuitions and help us keep our wits about us as we venture into ever more rarefied airs of abstraction.

One good picture comes to us by way of the field concept.  Given a space X, a field of a specified type Y over X is formed by associating with each point of X an object of type Y.  If that sounds like the same thing as a function from X to the space of things of type Y — it is nothing but — and yet it does seem helpful to vary the mental images and take advantage of the figures of speech most naturally springing to mind under the emblem of the field idea.

In the field picture a proposition f : X \to \mathbb{B} becomes a scalar field, that is, a field of values in \mathbb{B}.

For example, consider the logical conjunction pq : X \to \mathbb{B} shown in the following venn diagram.

Conjunction pq : X → B
\text{Conjunction}~ pq : X \to \mathbb{B}

Each of the operators \mathrm{E}, \mathrm{D} : X^\bullet \to \mathrm{E}X^\bullet takes us from considering propositions f : X \to \mathbb{B}, here viewed as scalar fields over X, to considering the corresponding differential fields over X, analogous to what in real analysis are usually called vector fields over X.

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Differential Logic • 13

Transforms Expanded over Ordinary and Differential Variables

Two views of how the difference operator \mathrm{D} acts on the set of sixteen functions f : \mathbb{B} \times \mathbb{B} \to \mathbb{B} are shown below.  Table A5 shows the expansion of \mathrm{D}f over the set \{ p, q \} of ordinary variables and Table A6 shows the expansion of \mathrm{D}f over the set \{ \mathrm{d}p, \mathrm{d}q \} of differential variables.

Difference Map Expanded over Ordinary Variables

\text{Table A5.}~~ \mathrm{D}f ~\text{Expanded over Ordinary Variables}~ \{ p, q \}

Df Expanded over Ordinary Variables {p, q}

Difference Map Expanded over Differential Variables

\text{Table A6.}~~ \mathrm{D}f ~\text{Expanded over Differential Variables}~ \{ \mathrm{d}p, \mathrm{d}q \}

Df Expanded over Differential Variables {dp, dq}

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