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Last updated 5 Sep 2005
Reformatted: 15 Nov 2017

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What's Wrong With
The Priority of Dynamical Systems Hypothesis

Notes for discussion
Aaron Sloman
http://www.cs.bham.ac.uk/~axs

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NOTE Added 16 Jan 2006:

Some of the ideas behind this web site are explained more clearly with examples in this web page on recombinable orthogonal competences.

The points are made in relation to a distinction between learning about two kinds of causation, Humean and Kantian in this presentation (on two views of a child as a scientist).

Related points are made in a partly overlapping presentation on A (possibly) new theory of vision.

Introduction: understanding structure, motion and causality

I am discussing with several people the possibility of a project that will be closely related to, but complementary to, CoSy. It is especially close to the PlayMate scenario in CoSy.

The new project will focus on trying to understand some of the exploration-driven learning mechanisms found in many animals, with particular emphasis on the development of understanding of spatial structure, motion and causality, as opposed to, for example, recognition, classification, prediction or tracking of objects.

The project is not primarily concerned with the probabilistic (Humean) notion of causality that is currently much discussed in AI and child psychology, which is concerned with correlations and how they are tested (as illustrated by Alison Gopnik's invited talk on children as scientists at IJCAI05, and in the last few papers available from her home page).

Another recent example of research in that field is Mark Steyvers, Joshua B. Tenenbaum, Eric-Jan Wagenmakers, Ben Blum, 'Inferring causal networks from observations and interventions' Cognitive Science 27 (2003) pp 453-489
http://web.mit.edu/cocosci/Papers/steyvers-etal-2003.pdf

In contrast with such work, our investigation of causation will focus mainly on learning to perceive, understand and deploy the deterministic causal relations involved in change of structure or change of relations between structures. Kant argued in his Critique of Pure Reason (1780) that Hume was wrong and that we have a conception of causation that goes beyond mere correlation and degrees of probability, but is deterministic. I believe Kant was right and that the ability to understand this notion of causation was a biological precursor to the development of mathematical capabilities. But exactly what the Kantian thesis is remains problematic: neither he nor Hume had conceptual tools for thinking about the design of a working system that learns about causation of either kind.

This is not to deny the importance of Humean concepts of causation: they are used either when we discover causal links that we do not understand (e.g. a child learning that flipping a switch can make a light go on or off), or when there are multiple causes for a particular event, e.g. age, strong winds and gravity combining to make a tree collapse. But the history of science shows that often what is initially thought of as Humean causation is later seen to have been ill-understood Kantian causation, where an underlying deterministic mechanism explains the observed correlations (e.g. fire producing smoke and ash from a pile of wood, or eating certain plants making people die).

It is sometimes argued that quantum mechanics shows that there is a deeper notion of probabilistic causation underlying the deterministic causation of physics, chemistry, biology, etc. But that is not an issue discussed here. (There is a kind of deterministic relationship between structure and long-run frequency that often underlies what is normally thought of as probabilistic causation, based on the mathematics of combinations and permutations, but that's a topic for another discussion.)

Examples of deterministic causation

Examples include the following familiar kinds of causal relationships.

• You can make a rod move away from you by pushing its end with a closed fist but not pull it towards you unless you first open and shut your fist so as to grasp the end.

• If one end of a rigid rod is behind a screen and the other end is pushed towards the screen, the hidden end will move on the far side of the screen, and if the visible end is pulled away from the screen, more of the rod will gradually become visible and the hidden end will eventually appear.

• You can use a length of (inelastic) string to pull something towards you but not to push it away (unless the string is tightly rolled in a ball).

• If you hold two ends of a piece of string, one in each hand, and loop the string round an object then you can pull the object towards you by moving both hands. But if you then let go of one end of the string, you can no longer move the object by pulling the string.

• A straight, smooth, piece of wire held at one end can be used to push an object away from you, but (usually) cannot be used to make the object move towards you. However if you bend the end of the wire (and it is inelastic so that it keeps its new shape) then the wire can be used to pull an object towards you.
  (Note: a precursor to understanding this may be understanding the causal differences between pre-existing straight and bent objects found in the environment.)

• If two toothed wheels (gear wheels) are in the same plane, on fixed parallel axles, with their teeth enmeshed as in the diagram, and both separately are free to rotate around their axles, then if one of the wheels rotates clockwise the other must rotate counter-clockwise.
  

  In contrast, if the wheels are linked by some mechanism hidden in a box

  one may discover that there is a relationship between the direction of rotation of one wheel and the direction of rotation of the other, but without understanding why, and without knowing whether the direction could change as as a result of some alteration in a hidden mechanism.

• If A, B, C and D are four rigid connected rods of varying length, lying in a plane in the form of a convex quadrilateral, with A linked at one end to B and the other end to D, B linked at one end to A and the other end to C, etc., as in the figure on the left, below, then if two of the "opposite" joints (e.g. the joint between A and B and the joint between C and D) are moved closer together or further apart, then the distance between the other two joints will increase or decrease. I.e. as one distance increases the other will decrease, and vice versa, while the shape remains convex.
  

• If the shape becomes concave (after two of the rods have momentarily formed a straight line), the causal relationship changes, e.g. if two opposite joints move closer then so will the other two, as in the figure on the right.

• If the linked rods are not in a plane, e.g. if the links are ball-and socket joints and the link where C and D meet is not in the plane, then that link can be moved closer to the opposite link without altering the distance between the other two links.

• If three rigid rods are loosely linked, the structure is rigid, whereas if one of the links is broken the three rods will be able to move relative to one another.

• A screw in a bolt must move relative to the bolt along its axis if the screw rotates around its axis, and the speed and direction of linear motion depends on the speed and direction of rotation.

I discussed our ability to understand and reason about spatial change using spatial forms of representation in http://www.cs.bham.ac.uk/research/cogaff/sloman-analogical-1971/, a paper presented to IJCAI in 1971, criticising the logicist philosophy of AI presented by McCarthy and Hayes, in 1969.

The list above presents just a tiny sample among thousands of facts about structure, motion and causality that every normal human child is capable of learning. Some of them are also learnt by some other animals. It is likely that nest-building birds and tree-climbing animals have to learn many examples that most human children do not normally learn, though they may be capable of learning some of them. Animals that fly may learn many things that humans never learn, e.g. about the fine-grained control of motion in flight, though some humans learn the underlying aerodynamical principles that the flying animals never learn!

Nothing is claimed at present about how such things are learnt. It is possible that in every case a child first learns the causal connections as mere correlations (Humean causation) and then later comes to understand the deterministic (Kantian) causal explanation of what is happening. Alternatively it may be that in some cases Kantian understanding can be immediately extended to new cases without having to go through an intermediate Humean understanding of those cases, e.g. seeing for the first time that a rigid circular plate (e.g. lid of a coffee tin) can be used to perform some of the same functions as a long thin lever, or perhaps seeing for the first time that bending a piece of wire to form a hook can enable you to make an object move towards you, as Betty the New Caledonian Crow did in Oxford, much to the surprise of researchers there in 2002.

Learning such things, or being given the information innately, will be required in a robot that can manipulate objects intelligently, for example in order to help a disabled person with everyday tasks in a kitchen.

However, fast, fluent, unintelligent manipulation is possible without understanding. One kind of ability, the ability to behave expertly, may be implemented in appropriate kinds of dynamical control systems, whereas the other kind, the ability to reason about what behaviour is possible and what its consequences will be is what we are concerned with here, and that does not necessarily go hand-in-hand with speed and fluency.

Causation in virtual machines

In the early drafts of the section about differences between Humean (probabilistic/statistical) causation and Kantian (deterministic) causation I forgot to include a very important example of the latter that we have only come to understand during the last half century, namely causation in virtual machines that we have designed, implemented, tested, debugged, and often deployed in important applications (e.g. internet shopping systems).

One reason why causation (usually, though not always deterministic) in virtual machines that we have built and understand well is important is that it at last helps us understand much older examples of causation in virtual machines, namely mental causation, which has been very puzzling to philosophers had scientists for a long time. For more on this see this online presentation (PDF).

I mention this here because it is relevant to how a child or animal might think about mental states and processes as causes, both within itself and in others, though this document is more concerned with physical causation. Elsewhere I have suggested that biological evolution solved the philosopher's 'Other Mind's' problems

How can I know that anyone else has a mind?

How can I conceive of others having mental states and processes when all I observe is physical behaviour?

in its own way --- by providing genetically determined mechanisms and forms of representation for use in understanding the behaviour of other animals in terms of things like perception, knowledge, desires, mental capabilities.

Whatever mechanisms are available for representing and reasoning about one's own mental states and processes, and causes therein, will also in principle, be usable for reasoning about others. (Some of the problems and difficulties to be overcome by such mechanisms are mentioned here in the section on hypothetical information and gaps therein.)

The details of such mechanisms need not all exist at birth, and it is very likely that they grow and develop during their use, but such growth requires some genetically determined starting point, which may be present in different forms with different potential in humans and other animals.

Initially human thoughts about mental processes in intelligent machines were something like metaphorical extensions of our ability to think about humans and other animals. But gradually, during the last 50 years or so we have acquired a new more solid understanding of virtual machines and causation therein, which is now used every day in designing, testing, debugging, explaining, and reasoning about ever more complex software systems many aspects of which, though not all, are deterministic.

The relevance of dynamical systems to understanding
(Doing and understanding are related but different)

Discussions with colleagues in the CoSy project and elsewhere have raised some questions about different sorts of requirements for understanding and manipulating objects in the environment. In particular, there are different architectural requirements for (a) physical control of movement and (b) understanding structures, processes and the relations between them.

• (a) requires control of low level dynamics, which is something every animal needs, in order to be able to walk, run, fly, pick things up, throw things, push things, pull things, eat, mate, etc. It involves constant online control of movement of parts of the body and things the body is in contact with, which can include dynamic (constantly changing) control of positions, linear and angular velocities, forces, acceleration, deceleration etc.

• (b) is a reflective grasp of structure and kinematics, that is, the sort of thing that allows you to use what you see to
  (1) predict what is going to happen, e.g. where two moving things will meet, whether something will or will not fit through a gap, whether something will break, whether something will be stable or fall over, etc.

  (2) plan actions in advance of performing them, including considering different ways of achieving something and choosing a good one,

  (3) explain how to do something: e.g. show the angle at which something must be held to fit into a slot, describe or show how big a gap needs to be for a particular thing to go through it, describe or show what shape something needs to be in order to perform some function (e.g. levering up the lid of a coffee tin),

  (4) explain how something came about, e.g. why something broke, how something was caused to move, why a collision did not happen, etc.

  (5) think about things that are not currently seen, e.g. where something must now be if it was moving at a constant speed and has gone out of sight behind something bigger, where someone foot is when only the upper half of the person is visible, say whether two people seen standing on the far side of a wall could be holding hands or not, etc.

In recent years there has been a lot of work on (a) (dynamic control) in robotics, e.g. in Rod Brooks' lab in the USA and other places. However it is very difficult to do and requires specialised robotic equipment with very sophisticated motors, sensors and sensory feedback, typically using force-control rather than position-control.

It is quite difficult to work on this without having mechanical engineering expertise and very expensive equipment.

However, it may be that that's not where we should be going if we are primarily interested in cognitive processes that are involved in the relatively rare, more sophisticated, animals that not only do things, but also can think or reason or communicate about what they do, i.e. have the reflective abilities of type (b).

For that we need to address different sorts of issues about forms of representation and reasoning which don't merely produce actions, but answer questions, make predictions, etc. They could be used in producing and controlling actions, but not necessarily with speed and fluency of the kind required for competence of type (a).

(Is this a distinction made clearly by psychologists and neuroscientists studying action? Personally I think that what Milner and others described as the difference between 'what' vs 'where' visual pathways was just confused, and was really concerned with the difference between vision used for tasks of type (a) and vision used for tasks of type (b).)

QUESTION:

Consider the following thesis, which we could call Priority of Dynamical Systems (PDS):

PDS:
It is not possible for a machine (or animal) to have reflective capabilities of type (b)
if it does not have an appropriate range of dynamic control capabilities of type (a).

I think there is a wide-spread belief that is now current at least among some AI researchers, and I suspect many others, that PDS is true. Some even seem to go further and believe that the possession of the dynamic control capabilities is *sufficient* for all the sorts of tasks that we think need planning, prediction, explanation, etc. (I believe Rolf Pfeifer is an extreme example and possibly Tim van Gelder, in BBS 1998). I think there are echoes of this in the current interest in 'mirror neurons'.

PDS implies that in order to be able to reason, predict, explain, etc. features of movements of a certain type one must have previously been able to produce similar movements. I.e. deliberative and reflective capabilities depend on dynamic control capabilities.

One reason for thinking this might be a belief that processes of type (b) require mechanisms that *simulate* the processes that are being thought about, where simulation involves somehow running through the same control processes without performing the actions overtly. This might be a result of using feed-forward (predictive control) mechanisms to provide simulated sensory feedback when movements are simulated internally. Recent work by Murray Shanahan is an attempt to implement this sort of thing: http://casbah.ee.ic.ac.uk/~mpsha/ShanahanAISB05.pdf

IS PDS TRUE?

I believe PDS is false, and that there is a kind of mechanism/architecture that at least some 'altricial' species with 'non-viscous' cognitive development, have which involves the ability to represent and reason about structures and processes which is very different in character from the mechanisms that are involved in physically controlling actions and which provides the ability to think about, explain, design, and indirectly control processes of types that one has never physically produced.

Kinds of evidence for the falsity of PDS include the following facts:

• People born limbless as a result of thalidomide or other causes of malformity do not seem to be incapable of thinking and talking about actions produced by others they have never performed themselves.

• Many humans think about acquiring skills before they actually acquire them, and they use their understanding of those skills to devise means of training or practising. Examples include doing things like performing somersaults, pole-vaulting, catching a ball (which a young child typically tries to do before it first succeeds), playing a musical instrument.

  If thinking about an action required the use of competence at achieving it then they could not think about doing anything, or understand an instruction or suggestion to do it, before being able to achieve it.

  (Has anyone ever attempted a survey of animal species, including insects, fish, crustaceans, etc., asking the question: how many things do the young try to do and fail and later succeed in doing by developing new ways of combining sub-actions?)

• Throughout history, many normal humans with engineering abilities have been able to think about processes that no human is able to produce directly, then design a machine that is capable of producing the process, or in some cases (e.g. construction of the Egyptian pyramids) design a plan involving large numbers of people and animals doing things in a coordinated way, to produce the result. All modern machines, buildings and vehicles are results of such capabilities.

• Evidence from brain damage .....

IMPLICATIONS OF PDS BEING FALSE

This section still to be written.

One implication is that if we are to test cognitive mechanisms that do not depend on the low level fluent control of physical movements, then in addition to 'question answering' tests we will need physical tests that do not necessarily involve impressive motion. It could be that everything is slow and laboured, like human patients whose low level motor control mechanisms have been damaged but who are still able to use cognitive sub-systems to control their movements.

This also means that our requirements for robot arms can be much simpler if we are not concerned with fast, fluent actions!
(Compare a normal human learning to control a mechanical digger by sitting in its cab and moving knobs and levers.)

NOTE on polyflaps

In connection with plans for designing an altricial robot that develops non-pre-configured competences of the sort discussed in
www.cs.bham.ac.uk/research/cogaff/05.html#200503
and
www.cs.bham.ac.uk/research/cogaff/05.html#200502
we have been trying to specify a domain in which the child-like robot could play and explore and make discoveries about causation, about things worth seeing and things to do. A sample domain in which manh aspects of Kantian causation is the domain of polyflaps, discussed here (with pictures):
http://www.cs.bham.ac.uk/research/projects/cogaff/misc/polyflaps

Comments and criticisms welcome.

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Aaron Sloman