Perceptual Forms of Movement and Naive Physics in Phenomenology

Kohler ([1917] 1927: 149) had already remarked that the knowledge explicitly formulated in physics has a kind of intuitive correlate in phenomenological form that is embedded in perception whose aim, however, is to fulfil the needs of a biological fit to the environment. The work of Bozzi (1958, 1959, 1961b) on the perception of pendular oscillations and of movements over inclined planes is a piece of phenomenology of this perceptual knowledge. Independently of common-sense and scientific knowledge, it regards the modes of appearances of movement in the environmental world. Beside the phenomenal quantita­tive properties, such as velocity and time, in the perceived movement, subjects are sensible to the form of movement and to qualitative features connected to velocity, such as a natural vs a forced movement. This perceptual dimen­sion may have no sense in physics, but it reasonably conveys a meaning on the nature of the movements of things. On the grounds of the work of Michotte, it can be expected that together with phenomenal structures of movements like launch or push there must be phenomenal “natural” movements with a speed that appears to be in accord with the nature of the things in motion rather than resulting from external forces. Michotte showed that for the perception of causality it is necessary that the second object, after the contact with the first, moves for a while along a path and then stops. The movement seen as the natural consequence of being hit is at odds with the moment of inertia.^[1]

Likewise, subjects seem sensible to slow and fast movements between which there should be a class of movements with a perceptually “natural” speed. Such a movement does not make sense in physics, although it is possible to give a physical definition of it. If S is an immobile reference system, two systems S₁ and S₂can be chosen that are in relative uniform motion with respect to S, so that the relative velocities between S₁ and the rest define the class of slow motions, while those beyond S₂ define the class of fast motions. The relative velocities of the systems falling between S₁ and S₂define the natural motions, in the sense that they are neither slow nor fast, thus having a “just” velocity. Yet this definition does not capture the meaning of movements that are perceived as natural, despite the fact that in a great deal of cases of ordinary experience this character is directly forced upon subjects as a feature of those movements that are exclusively due to the thing in motion.

Bozzi tries to obtain an ostensive definition of perceptual natural move­ments. In a first experiment, a bar 40 cm long in pendular motion with a range of frequencies of 17-95 arc minutes with a 10° amplitude is presented to subjects. Only one frequency corresponds to the frequency that the bar should physically have after being left free to oscillate. Subjects are asked to say which frequency is “just” or natural for the oscillation of the bar. At the physically right frequency (56 oscillations per minute), the bar appears to swing too fast. In general, anything above 50 oscillations per minute is per­ceived as too fast. Below 30 oscillations per minute, the bar is perceived to swing too slow. The frequency at which the bar appears to swing at the natural speed corresponds to about 40 oscillations per minute. Bozzi (1959) empha­sizes that the single values at which the mode of appearance of the oscillatory motion changes are different across subjects, but the relations among the three classes of velocities and their order is preserved within and among the subjects. It is interesting that this ordering of movements into three phenom­enal regions with transitions through different clear-cut perceptual characters has the same structure found by Wertheimer and Goldmeier for the regions of possible appearances of movements and of similarity. Indeed, the perceptual character of natural movement is displayed in appearances that occur and disappear through a smooth transition from perceptually slow movements to perceptually fast movements. Therefore, Bozzi infers that it is not derived from past experience with things that display a harmonic pendular motion. Rather, it is a mode of appearance that is part of the movement according to the oscillation amplitude, like form or weight is part of a perceived thing. If the amplitude is increased to 60°, the frequency at which the oscillation ap­peared natural brings about the perception of fast movement. Bozzi finds that for increasing amplitudes of the oscillation, the frequency corresponding to a natural swing has to be decreased and vice versa; hence, the law of isochrony does not hold for the perception of a natural pendular movement. Rather, the greater the amplitude of the oscillation, the slower the bar is perceived to oscillate.

In a second experiment, subjects are asked to adjust the speed of an ap­parent movement of a figure on an inclined plane in order to perceive a natu­ral movement so that the figure appears to slide rather than being braked or launched (Bozzi, 1959). As for natural oscillation, subjects find that this task makes sense. Bozzi presents two squares, with sides of length 4 mm and 16 mm, moving downwards on the 30 cm side of a triangle for a 5°, 10°, 22°, 45° and 68° inclination. The path on which the squares slide is divided into three or four sections. Subjects are also asked to point to the section where the move­ment of the square appears natural. A uniform motion is seen as too fast at the beginning of the path. A naturally accelerated motion is seen as too fast at the end. Natural movement is perceived only for some sections of the path at particular combinations of the variables. If the small square slides along a 45° inclined plane with uniform motion at a speed of a few cm/sec., the whole movement appears braked and subjects report seeing friction. If the speed is increased, the square appears to be launched rather than gliding. If the mo­tion takes a second to be completed, the square appears to move naturally for the last two-thirds of the path, while it appears to jolt in the first third. If the motion is uniformly accelerated, as should be expected for a physical motion on an inclined plane without friction, and it takes no more than one minute, the square appears to move naturally for the first section of the path and to be launched for the last two-thirds. Thus, the phenomenological form of natural movement must be such that the object is accelerated in the first section and, after reaching an appropriate speed, moves uniformly till the end of the path on which it travels. To ascertain the autonomous phenomenological nature of this form of movement, subjects are asked to adjust the variables of the incli­nation and the size of the object travelling on it. By bringing about smooth transitions of the perceived values of these variables, the condition in which the optimal appearance of such a natural movement occurs or disappears can be determined. Bozzi finds that if the optimal appearance takes n centiseconds and the inclination of the plane is decreased from 45° to 22°, the time the ob­ject takes to complete the path must amount to m seconds for m > n. If the size of the object is linearly doubled, the time the object takes to complete the path must amount to m seconds for m < n.

Bozzi claims that these results point to a system of functional connections that allow one to understand the modes of appearances of the movement of things. He contends that the functional connections may be regarded as a piece of phenomenological naive physics. Indeed, these results are an antecedent of naive or intuitive physics (Shanon, 1976; McCloskey et al., 1983; Bozzi, 1989; Smith and Casati, 1994). However, they do not imply that an intuitive or pre­scientific theory is embedded in the subjects’ minds, rather that the perception of movement is a kind of a phenomenological module of experience that is independent of physical knowledge or learning on the grounds of past experi­ence. It regards the phenomenal structures in which the form and the meaning of movement and important properties of moving things are tied together. The phenomenal structures are at the same time a piece of the phenomenological theory of movement and an observable regularity of the perceivable world. Some pieces of this theory have a model in structures that are consistent with physics: the more inclined the plane, the faster the movement of an object on it. Some pieces, meanwhile, do not: the bigger the object, the faster it moves on the inclined plane, so that the time to complete the path must be shorter for the movement to appear natural.

Source: Calì Carmelo (2017), Phenomenology of Perception: Theories and Experimental Evidence, Brill.