Argue that the theory of common sense structures provides an important and hitherto unappreciated link between early Gestalt psychology on the one hand and contemporary developments in philosophy and in artificial intelligence research on the other.

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Introduction 

In the works of Aristotle or of the medievals, as also in the writings of later common-sense philosophers such as Thomas Reid or G. E. Moore, we find a family of different attempts to come to grips with the structures of common sense and of the common-sense world that is given to us in normal, pre-theoretical experience. We shall argue in what follows that the theory of such structures provides an important and hitherto unappreciated link between early Gestalt psychology on the one hand and contemporary developments in philosophy and in artificial intelligence research on the other.

The notion of providing an adequate theory of the common-sense world has been taken seriously of late above all by those, such as Patrick Hayes or Kenneth Forbus, who see in such a theory of what they call `naive' or `qualitative physics' the foundations of future practical successes in robotics.(2) This naive physics is, however, like cognitive science in general, in a state of flux, and a serious philosophical investigation of its presuppositions and achievements has hardly been attempted. Yet it is already at this stage possible to point to a certain apparent defect or one-sidedness of current research in this field that is due to the predominant assumption that it is set theory and related instruments of ontology that are to provide the basis for naive-physical theorizing. The defect arises, we shall suggest, in virtue of the fact that naive physicists working in the A.I. sphere are for obvious reasons concerned with certain specific sorts of formal implementations. Their motivations are in the first place pragmatic, and so their aim is not so much a theory of the common-sense world that could be defended as being true, but rather a theory that has certain sorts of practical advantages from the point of view of implementation. Both of these factors, we shall argue, lead the naive physicists to neglect important detailed contributions to the theory of common sense that have been made by both psychologists and philosophers, contributions which it will be the business of the present paper to describe.

What will be surprising to those who are acquainted mainly with the standard artificial intelligence literature on the topic of naive or commonsensical physics is the extent to which it is among the Gestalt psychologists, above all, that some of the most important and original work in this respect is to be found. Indeed one could argue that the Gestalt-theoretical approach to external reality is in its entirety a variety of naive physics, something which is brought out clearly for example in the pronouncements of Wolfgang Köhler to the effect that there seems to be `a single starting point for psychology, exactly as for all the other sciences: the world as we find it, naïvely and uncritically'. Our naive experience, as Köhler points out, `consists first of all of objects, their properties and changes, which appear to exist and to happen quite independently of us' (1947, p. 1, 2). We can compare, on this very issue, Gibson:

Some thinkers, impressed by the success of atomic physics, have concluded that the terrestrial world of surfaces, objects, places, and events is a fiction. They say that only the particles and their fields are "real" . . . But these inferences from microphysics to the perception of reality are thoroughly misleading. The world can be analyzed at many levels, from atomic through terrestrial to cosmic. There is physical structure on the scale of millimicrons at one extreme and on the scale of light years at another. But surely the appropriate scale for animals is the intermediate one of millimeters to kilometers, and it is appropriate because the world and the animal are then comparable. (1966, p. 21f)

It is this intermediate world, the world of common sense, which will be our concern in what follows.

I. GESTALT THEORY AND THE HISTORY OF NAIVE PHYSICS 

Avenarius and Mach 

A full treatment of theories of common sense would have to deal with Aristotle and the Scholastics, with work on early physics in the spirit of Pierre Duhem, with Galileo's presentations of the received physical theory he then criticizes, with Thomas Reid and the Scottish school of common sense. Our story, however, shall begin with Richard Avenarius and Ernst Mach, both of whom explicitly sought a view of the world as this is directly given in perception. More precisely, Avenarius and Mach operate with a notion of `pure perceptions', which is to say, perception conceived as having been stripped of those metaphysical ingredients (for example ideas about absolute space and time) which, as they conceived matters, are illegitimately imported into our experience. The `natural concept of the world', on Avenarius' view, `is that general concept we all have about the world in its entirety before any exposure to philosophy':

Intrinsic to the natural concept of the world is the unshaken belief that all the component parts of my environment exist and develop, change or remain constant, in interaction with one another, in some form of stable regularity, all independently of my observing them or not observing them. (Scanlon 1988, p. 220f.)

The matter was approached from another, complementary standpoint by Mach, who sought to found the science of physics on a purified world-view along lines similar to those considered by Avenarius to show how physical science can grow, as it were organically, out of common-sense experience. Mach's ideas in this respect formed part of the background of Einstein's work on the theory of relativity, but the biological approach to knowledge which Mach and Avenarius shared the idea that theory should evolve naturally from out of the ground of ordinary experience anticipated also the work of the Gestalt psychologists and certain aspects of the ecological ideas we find in Gibson.

Like all their forerunners, however, neither Mach nor Avenarius has any notion of a separate discipline of naive physics of the sort that is at issue in current artificial intelligence research. Moreover both allowed their respective images of reality to become infected with doctrines of elementarism and of neutral monism of a metaphysical sort. Thus their attempts to protect their views of `pure perception', etc., against alien impurities were not, in the end, completely coherent.

Köhler, Lipmann, Bogen 

Wolfgang Köhler was influenced both directly and indirectly by Mach,(3) and it is in fact in the correspondence of Köhler that there appears what is perhaps the first occurrence of the term `naive physics'.(4) In his The Mentality of the Apes, a work whose original German text dates back to 1917, Köhler points out that 

psychology has not yet even begun to investigate the physics of ordinary men [Physik des naiven Menschen], which from a purely biological standpoint, is much more important than the science [of theoretical physics] itself (Köhler 1921, p. 149).

As Köhler makes clear: 

not only statics and the function of the lever, but also a great deal more of physics exist in two forms, and the non-scientific form constantly determines our whole behaviour. (With experts, of course, this is saturated in all stages by physical science in the strict sense.) (Köhler, loc. cit.)

Köhler's ideas were worked out in detail in application to the different levels of intelligence manifested by school-children by the Berlin Gestalt psychologists Otto Lipmann and Hellmuth Bogen in a work entitled Naive Physik, published in 1923. The latter comprises first of all a theoretical investigation of the nature and scope of naive physics itself, which is seen by Lipmann and Bogen as a capacity for intelligent action in relation to everyday tasks and objects. There follows then a summary of the results of experimental work on naive-physical beliefs about causality and natural law and on the relation between such beliefs and corresponding actions of children of different levels of intelligence. 

Interestingly, Lipmann and Bogen see naive physics as a true, and therefore useful, discipline. Thus they argue that children should be trained in naive physics (where many later psychologists have been interested, rather, in those naive-physical beliefs which stand out as being false).(5) 

Gibson 

J. J. Gibson's investigation of the world of basic affordances for human action consists in the attempt to establish a new descriptive standpoint which would pick up `facts at a level appropriate for the study of perception'. Such a level is prima facie set against standard mathematical physics and related disciplines which are concerned with `the atomic and cosmic level of things' and leave out everything in between.(6) 

Gibson, however, is confident that these intermediate level facts `are consistent with physics, mechanics, optics, acoustics, and chemistry', being only `facts of higher order that have never been made explicit by these sciences and have gone unrecognized'. (1986, p. 17) Hence in contrast to the position of Galileo or Locke as concerns the world that is given in common-sense experience it is possible to develop a realist theory of the given facts, and this in a manner which does not involve the rejection of standard quantative physics. Gibson terms `ecology' the discipline that should encompass these higher-order facts; it is presented as `a blend of physics, geology, biology, archeology, and anthropology, but with an attempt at unification' on the basis of the question: what can stimulate the organism? (1966, p. 21) 

Gibson acknowledges a `debt to the Gestalt psychologists, especially to Kurt Koffka' whose ideas in shaping this new, intermediate level of description Gibson sees himself as having extended (in this connection he mentions also Katz, Michotte, Hochberg, Metelli and Johansson). This Gestaltist setting shows up most interestingly in Gibson's nomenclature for surface layout, whose basic concepts we shall meet again in our thematic section below. These concepts apply to what Gibson calls surface geometry, a discipline which stands to naive physics in something like the same relation in which the more familiar varieties of abstract geometry stand to physics in the standard quantitative sense.(7) 

The Austro-Italian School of Gestalt Theory 

In some respects parallel to the Berlin Gestalt tradition of Köhler and Wertheimer is the work of the Graz school around Alexius Meinong (under whom Ehrenfels had studied in Vienna). Meinong's own "On the Origins of our Knowledge in Experience" of 1906 contains ideas on the world of external perception which then influenced Fritz Heider's work on "Thing and Medium" to be discussed below. The Graz school was translated through Meinong's assistant Vittorio Benussi to Italy, where subsequent generations of Gestalt psychologists have made a number of serious contributions to our problem.(8) 

Some of the very first experimental work on naive physics was performed by the Italian Gestaltist Paolo Bozzi in the late '50s, in a manner which recalls earlier work by Michotte on the perception of causality. Bozzi's subjects were asked to select, from a range of more or less artificially constrained cases, which movement of a pendulum looked most natural. As he recalls in his autobiographical book Naive Physics,(9) such experiments were partly inspired by a study of the naive conceptions of the Aristotelian spokesman Simplicio in Galileo's Dialogue, Bozzi's idea being that these earlier `naive' views of physical reality reflect or are influenced by the ways in which we are disposed perceptually to organize the physical reality we see.(10) There is, according to Bozzi, a holistic interwovenness of experienced qualities of objects and of certain physical conceptions we have of them. Qualities such as force, agency, resistance, harmony, equilibrium, etc. can thus be seen to play a primary role in perceptual organization. 

Phenomenology 

The phenomenologists, too, concerned themselves in systematic ways with the idea of a science of common-sense experience. As the cultural anthropologist R. M. Keesing points out (1987, p. 375): `much of phenomenology (Husserl, Schutz, Heidegger, Merleau-Ponty) is precisely about models of everyday cognition'. Further, it is well known that there was an important interplay, both personal and theoretical, between the phenomenological and Gestaltist movements, whose respective members shared also a wide spectrum of common sources. Our investigations here will to some extent be concentrated in the areas of this interplay and more precisely still in relation to the work of Husserl, Schapp and other early phenomenologists. A complete treatment would however need to consider also the important contributions of Merleau-Ponty and of Heidegger to our understanding of the phenomenology of common sense. 

It is Husserl's Crisis of European Sciences, above all, which addresses in explicit philosophical fashion the problem of the relation between the ontology of the common-sense world called by Husserl the `theory of the structures of the life-world' and pre- and post-Galilean physics. As Husserl points out (Crisis, p. 65), one reason for the neglect of the naive and of the commonsensical in the history of philosophy has been that, due above all to the influence of Plato, philosophy wanted always to be episteme, and not doxa, turning up its nose at the latter not merely because it is unscientific but also (with less apparent justification) because it is not itself capable of serving as the object of a scientific treatment. The task of phenomenology, now, Husserl sees as being that of harmonizing the naive and the exact (of understanding the relation between the common-sense world and its various outgrowths and extensions in particular in the realm of science). In our thematic treatments of different sub-areas of naive physics below we shall return to Husserl's view of things, cause and change, and consider also the contributions of other thinkers on the borderlines of phenomenology and Gestalt psychology to our understanding of the detailed structures of naive-physical reality. 


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II. NAIVE PHYSICS AND ARTIFICIAL INTELLIGENCE

Historical Antecedents 

The basic approach of the A.I. naive physicists is to take a sampling of deductive inferences from a given domain and to see how formal languages can be developed in which the relevant knowledge can be axiomatically expressed and the relevant inference procedures formally captured. Such work has historical antecedents in the so-called `mathematical philosophy' that was initiated by Whitehead in a ground-breaking paper entitled "On Mathematical Concepts of the Material World" and published in 1906. This mathematical philosophy was then pursued in some of the early writings of Bertrand Russell, but ...

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