Reviewing the Ontological Problem of Knowledge through the Double-Slit Experiment

Paul Tillich says, “Knowing is a form of union. In every act of knowledge the knower and that which is known are united; the gap between subject and object is overcome. The subject ‘grasps’ the object, adapts it to itself, and, at the same time, adapts itself to the object” [1]. To understand the elements involved in knowledge, we will use the analogy of the double-slit experiment from modern physics in this essay.

In the double-slit experiment, the particles of light, called photons, are shot one at a time towards a screen. On their way to the screen, photons pass through two tiny slits. Figure 1 depicts the configuration of this experiment.

Assuming that photons are like tennis balls, we would expect that the balls pass through either the left slit or the right slit before hitting the screen. Thus, the marks of the balls on the screen should form two parallel stripes. However, the experiment shows that the patterns formed by the photons are a band of stripes. This is what you would expect to see if a constant beam of light shined through the two slits (see Figure 2).

How can single, bullet-like particles in Figure 1 form the same patterns as those in Figure 2? This could only happen if each photon in Figure 1 passes through both slits at once. To find out what really happens, we add photon detectors to the experiment in Figure 1. The photon detectors allow us to identify which slit the photon passes through before hitting the screen (see Figure 3). Surprisingly, though, adding the photon detectors changes the behavior of the photons to particle patterns; the wave patterns disappear and, instead, only two parallel stripes appear on the screen. When the detectors are removed, the wave patterns return. How could installing photon detectors change the patterns? This question has puzzled scientists for decades. The modern explanation is that the photon's path is not part of reality.

The rationale behind the patterns in the double-slit experiment is the object that the scientist, i.e., the subject, seeks to know. It is only through knowing this rationale that the subject and the object can be united. Tillich says, “But the union of knowledge is a peculiar one; it is a union through separation. Detachment is the condition of cognitive union. In order to know, one must ‘look’ at a thing, and, in order to look at a thing, one must be ‘at a distance.’ Cognitive distance is the presupposition of cognitive union” [1]. Let us consider an imaginary scenario to understand this better. Suppose that in the double-slit experiment the photons have an unknown characteristic that allows them to truly predict whether they are being watched when passing through the slits. This unknown characteristic prevents the scientist from detaching from this phenomenon and, as a result, it prevents the knowing from occurring. In this case, the knower does not have the upper hand over that which is about to be known; therefore, cognitive union (which presupposes some equality) is the dominant element in the process of cognition. In circumstances where the knower has the upper hand, the distance element (which presupposes some inequality) is the dominant element in the process of cognition. Tillich stresses that the presence of both union and distance in the process of cognition results in knowledge.

Paul Tillich highlights two types of knowledge based on the dominant role of union and distance. The type of knowledge which is predominantly determined by the element of detachment is called controlling knowledge. The vast majority of what we learn at school is of this type. Our imaginary scenario for the double-slit experiment fails to meet the element of detachment, so the scientist cannot contribute to this type of knowledge. The second type of knowledge is called receiving knowledge. The following paragraphs will discuss this further.

In our imaginary scenario, the photons can accurately predict whether there are detectors attached to the slits and, as a result, change the patterns on the screen accordingly. In the case that our imaginary scenario corresponds to reality, there is no reason for us to not assume that sooner or later scientists will discover the scientific basis for this phenomenon, and the mystery surrounding our imaginary scenario will be revealed. However, when it comes to man’s essential nature, there are mysteries that, Tillich believes, cannot be scientifically figured out even though they work and produce intuition for him. Tillich says, “A truly objective relation to man is determined by the element of union; the element of detachment is secondary. It is not absent; there are levels in man’s bodily, psychic, and mental constitution which can and must be grasped by controlling knowledge. But this is neither the way of knowing human nature nor is it the way of knowing any individual personality in past or present, including one’s self. Without union there is no cognitive approach to man. In contrast to controlling knowledge this cognitive attitude can be called ‘receiving knowledge’. . . . Receiving knowledge takes the object into itself, into union with the subject. This includes the emotional element, from which controlling knowledge tries to detach itself as much as possible” [1].

In Figure 3, the scientist is surprised by the fact that adding the detectors to the double-slit experiment affects the wave patterns on the screen; this emotional reaction is a necessary step toward the unity of union and detachment. According to Tillich, subject and object cannot be united without emotional participation. He corresponds the unity of union and detachment to the term “understanding”. In other words, the emotional element of cognition is the building block of man’s understanding.

Receiving knowledge is particularly significant in disciplines whose knowledge is verified partly by experimental tests and partly by participation in the individual life with which they deal. Physicians, psychotherapists, educators, social reformers, political leaders, and historians come out of such disciplines. Life-processes are the object of such disciplines. In contrast to isolation, regularity, and generality which are presupposed by the experiments performed in the realm of controlling knowledge, life-processes have the character of totality, spontaneity, and individuality. This is why cognitive union plays an instrumental role in the aforementioned disciplines.

If the unknown characteristic of the photons in our imaginary scenario had the character of life-processes, receiving knowledge would play a significant role when it comes to knowing it because, then, understanding this unknown characteristic would be amalgamated with understanding the mysteries that shape man’s life; it would have the character of totality, spontaneity, and individuality. In situations like this, knowledge by participation comes to one’s rescue in the form of intuition. Tillich says, “Intuition in this sense is not irrational, and neither does it by-pass a full consciousness of experimentally verified knowledge” [1].

The concern that arises here is: Controlling knowledge comes with repeatable techniques for verifying it; how could receiving knowledge be verified if it is attained from an object with the character of totality, spontaneity, and individuality? Is this type of knowledge verifiable as well? The answer is if something is not verifiable, it is not knowledge. However, the territory of verification methods is not restricted to those used for controlling knowledge which Paul Tillich calls the experimental method. For receiving knowledge, verification can occur within the life-process itself. Tillich calls this type of verification experiential. He says, “Verification of this type (experiential in contradiction to experimental) has the advantage that it need not halt and disrupt the totality of a life-process in order to distill calculable elements out of it (which experimental verification must do). The verifying experiences of a nonexperimental character are truer to life, though less exact and definite” [1]. Receiving knowledge, he elaborates, “is verified by the creative union of two natures, that of knowing and that of the known. This test, of course, is neither repeatable, precise, nor final at any particular moment. The life-process itself makes the test. Therefore, the test is indefinite and preliminary; there is an element of risk connected to it. Future stages of the same life-process may prove that what seemed to be a bad risk was a good one and vice versa. Nevertheless, the risk must be taken, receiving knowledge must be applied, experiential verification must go on continually, whether it is supported by experimental tests or not” [1].

Continuing our imaginary scenario, let us assume that the photon has the characteristics of a living organism. How could the scientist verify his intuition on the predicting capability of the photons in this case? The answer to this question is not that straightforward. The scientist and the photon are both affected by each decision the scientist makes about the experiment when the photon has life-process characteristics. The scientist will never know the whole truth behind the experiment. However, his raging thirst for knowledge leads him to certain experiences and surprises along the way.

Tillich writes, “Knowledge stands in a dilemma: controlling knowledge is safe but not ultimately significant, while receiving knowledge can be ultimately significant, but it cannot give certainty” [1]. The threatening character of this dilemma emphasizes that if preliminary and incomplete verifications are not applied, it can lead to a “desperate resignation of truth”, namely occasions where man is dehumanized and led to confusion. The book Immunity to Change [2] has practical examples for such occasions.

References

1. Tillich P. Systematic theology. Vol 1: University of Chicago Press; 1951.

2. Kegan R, Lahey LL. Immunity to change: how to overcome it and unlock potential in yourself and your organization. Boston, MA: Harvard Business Press; 2009.