With this concept of attitude as a readiness to carry out a structural process, we have explained a number of facts hitherto ascribed to attention; which means that we have been able to replace a non-specific, [p. 550] ill-defined cause by one which is both specific and well-defined. The explanation is also consistent with the rest of our theory, and this consistency of the descriptive with the functional concepts employed should not be overlooked. A stepwise phenomenon, descriptively observed, and a stepwise process, functionally deduced, are thus brought into intimate connection. The structural process prepared by the attitude functions during the presence of a phenomenon as its physiological correlate, and this physiological hypothesis is determined by psychological observation; for we maintain that the physiological processes which underlie the structural phenomena must themselves possess the character of structures. This may seem to be a problem rather than a solution, but we shall presently see that even this problem has been successfully attacked.

Let us, therefore, turn to the experience itself. Upon a black cloth two squares of gray cardboard lie side by side. I am to judge whether or not they are of equal grayness. What is my experience? I can think of four different possibilities. (1) I see on a black surface one homogeneous gray oblong with a thin division line which organizes this oblong into two squares. For simplicity's sake we shall neglect this line, although it has varying aspects. (2) I see a pair of "brightness steps" ascending from left to right. This is a very definite experience with well-definable properties. just as in a real staircase the steps may have different heights, so my experience may be that of a steep or a moderate ascent. It may be well-balanced or ill-balanced, the latter e.g. when there is a middle gray on the left and a radiant white on the right. And it has two steps. This must be rightly understood. If I say a real stair has two steps, I do not say there is one plank below and another plank above. I may find out later that the steps are planks, but originally I saw no planks, but only steps. Just so in my brightness steps: I see the darker left and the brighter right not as separate and independent pieces of color, but as steps, and as steps ascending from left to right. What does this mean? A plank is a plank anywhere and in any position; a step is a step only in its proper position in a scale. Again, a sensation of gray, for traditional psychology, may be a sensation of gray anywhere, but a gray step is a gray step only in a series of brightnesses. Scientific thought, concerned as it is with real things, has centered around concepts like "plank" and has neglected concepts like "step."[7] Consequently the assertion has become true without qualification that a "step" is a "plank". Psychology, although it is [p. 541] concerned with experiences, has invariably taken over this mode of procedure. But since the inadequacy occasioned by the neglect of the step-concept is much more conspicuous in psychology than it is in physics, it is our science that first supplied the impulse to reconsider the case. And when we do reconsider, we see at once that the assertion "a sensation of gray is a sensation of gray anywhere" loses all meaning,[8] and that the assertion that a real step is a plank is true only with certain qualifications.

3. Attention: It is a recognized fact, that, clear and simple as association and sensation appear to be, there is a good deal of obscurity about the concept of attention.[4] And yet, wherever there is an effect that cannot be explained by sensation or association, there attention appears upon the stage. In more complex systems attention is the makeshift, or the scapegoat, if you will, which always interferes with the working out of these other principles.

[1.] Sensation: All present or existential consciousness consists of a finite number of real, separable (though not necessarily separate) elements, each element corresponding to a definite stimulus[1] or to a special memory-residuum (see below). Since a conscious unit is thus taken to be a bundle of such elements, Wertheimer, in a recent paper on the foundations of our new theory, has introduced the name "bundle-hypothesis" for this conception (65). These elements, or rather,-some of them, are the sensations,[2] and it is the first task of psychology to find out their number and their properties.

The conclusion we draw from these facts is not that Müller's theory is altogether wrong; for when we discard from it the concepts of apprehension, judgment, imagination and association [18], the competition among the two or three components remains. Only we would refer their effect to the general spatial level and not directly to the line. The components, therefore, find a place in our system as functional but not as descriptive facts.

To infer how something looks when it is not observable from the data of its appearance when at the crest of the attention-wave, means the acceptance of the constancy-hypothesis and a final abandonment [p. 561] of every effort to obtain a factual verification. As Köhler has pointed out (34), if we stand by description proper, i.e., by verifiable description, we must recognize that the T's have ceased to exist the moment we see the leaves, and that the T-phenomenon has been replaced by a totally different ground-phenomenon, which corresponds to the same part of the stimulus-complex. We see now what an enormous change has been effected when a figure "emerges" from its ground.

In Titchener's report we recognize the typical attempt of traditional psychology to elucidate phenomena by means of the cardinal concepts stated at the beginning of this paper. Something which ought to be there phenomenally, since a corresponding stimulus does exist, is not observable, and this contradiction is overcome with the aid of attention.

In absolute-threshold experiments we do not work with stepwise phenomena, as we do in differential limens, for our experience oscillates between one of a uniform ground alone, and one of a quality that stands out from the ground. Our assimilative phenomenon of the "level" which lies at the basis of all quality-judgments in the differential tests, is different from what we now call a pure ground experience. The "level" phenomenon is always experienced with a figure lying on a ground, and although the figure itself may be inarticulated, it is nevertheless distinct from its ground. The difference between absolute and differential thresholds is therefore well-founded, and our principles of structure enable us to comprehend it fully. The distinction is also corroborated by experiments which indicate that the two function quite differently. Specht (59) has shown that alcohol lowers the absolute and raises the [p. 556] differential threshold, and we can infer from this a functional difference between the two structures-the one, a figure against a ground, and the other, a part against another part of a figure.

Our conception has now been further enlarged; for while our deductions are in no wise dependent upon physiological assumptions, they are found applicable to purely physicochemical facts. We may therefore accept the fact that structures exist also in the realm of inorganic nature.

We can now state the structural theory of the Weber-Fechner law. The logarithmic law does not refer at all to single sensations, but to the whole structure; and from our deduction we must even [p. 552] infer that the concentration of ions in one area is a linear function of the intensity of the stimulus. Furthermore, what psychologists have called the process or function of comparing is not a third or "higher" factor accruing to the two sensations compared, but a moment inseparable from the whole structural system, which has been falsely singled out, just as the sensations have been falsely separated. In truth, comparison is always determined by a system in which one step necessitates another.

The term "potential difference" instead of misleading us, ought to furnish a striking analogy to our physiological stepwise phenomenon; for just as the step is a step only in a scale, so here each area has its potential only by virtue of the system in which it occurs, and just as the "upward (downward) direction " of the scale is a central property of the experience, so here the leap of potential is a central factor of the optical function.

From these two laws we can infer that the "goodness" of the scale has also a maximum or upper limit. Therefore, with an increasing stimulus-difference the step-height-experience will become less and less emphasized until an indifference point is reached, where the objective and the phenomenal difference coincide. At this point the emphasis will be replaced by an assimilative leveling, since the phenomenal difference has become less than the real one. If in a real scale we raise the height of the steps more and more we come at length to the point where we no longer have a scale. Two planks at levels ten meters apart are no longer two steps, and the same thing may happen on the phenomenal side. From the chirping of a cricket to the thundering of a sixteen-inch gun there is no scale, for they cannot be compared in the same sense in which we compare two strokes of a hammer.

Our theory finds confirmation in a crucial experiment, which shows, moreover, that these simple structures, far from being a peculiarity of the human species, are a very primitive form of reaction. As the question is put by Köhler (36), if an animal is confronted with two stimuli and is trained to react positively to the one and negatively to the other, what has it learned? The traditional theory would reply: the animal has formed a connection between the one sensation corresponding to the first stimulus and the positive reaction and likewise between the other sensation and the negative reaction; our theory, however, would say that the animal has learned to react to a certain structure.

Now Cornelius excludes the constancy-hypothesis from his theory. He assumes that to a single definite stimulus there corresponds, not a single definite sensation, but one of a number of several different ones (he denies also the continuity of the sensation-series). His theory therefore implies the general rule that sensation is not a function of the stimulus alone, and again it is attention that determines which of the many possible sensations will be aroused.

To understand the second attempt, made by Cornelius (4), we must analyze the "friction" and the "chance-error" hypotheses in their interpretation of Stumpf's paradox. Stumpf introduced his "function of perceiving" in order to avoid a contradiction. If a=b and b=c, it is contradictory that a ‡ c. However, the whole argument rests upon a tacit assumption.

Two attempts have been made to overcome this difficulty. The first is G. E. Müller's theory of the "chance-error" (43) which maintains that the final result of a stimulus is never the effect of this stimulus alone, for there are external or internal processes always at work to modify either the sensation itself or our apprehension of it. (In so far, Müller's theory is in harmony with Stumpf's unnoticed sensations.) Therefore it may well happen that though a > b, a-DELTA < b+DELTA. According to Müller, one of the causes of these chance processes is attention.

Methodologically the physiological and the psychological aspects of these three principles are in perfect harmony; the cortex has been divided into areas, the immediate experience has been analyzed into elements, and connections are assumed to exist between brain areas as between the elements of consciousness. Furthermore, the nervous processes may be altered functionally and their corresponding psychological elements are subject to the functional factor of attention. Evidently the psychological and the physiological are interdependent, and are not sensation, association, and attention, factual?

When I speak of perception in the following essay, I do not mean a specific psychical function; all I wish to denote by this term is the realm of experiences which are not merely "imagined," "represented," or "thought of." Thus, I would call the desk at which I am now writing a perception, likewise the flavor of the tobacco I am now inhaling from my pipe, or the noise of the traffic in the street below my window.

The Gestalt-Theorie is more than a theory of perception; it is even more than a mere psychological theory. Yet it originated in a study of perception, and the investigation of this topic has furnished the better part of the experimental work which has been done. Consequently, an introduction to this new theory can best be gained, perhaps, by a consideration of the facts of perception.