link. In other words, it seems that an innate tendency to integrate simultaneous and successively induced perceptions leads us to an awareness of a causal relationship. The achievement of insight into cause and effect brings a feeling of satisfaction and relieves psychic tension. A simple example may illustrate this psychophysiological assertion. From my desk I see on the horizon a dark bank of clouds coming nearer and nearer. Suddenly lightning from the cloud strikes the earth. A little later I hear the thunder.Momentarily ignoring earlier experiences of this nature, I am confronted with a visual, followed by an auditory, experience. The two phenomena manifest themselves independently of one another. Continuing to watch, out of curiosity, I see after some time another flash of lightning and hear again, later, a clap of thunder. Repetitions of these essentially identical sensory experiences lead inescapably to the interpretation that the optical and the subsequent acoustical phenomena are somehow related. With increasing frequency of repetition analogous successions are established as associative links, so that the conjecture of a causal relationship ultimately assumes the character of a certainty. Strictly speaking, no certainty exists but, at best, a high degree of probability. In everyday life one is, to be sure, surprisingly ready to assume a causal relationship. Obviously such a “short circuit” ordinarily suffices as a basis for adaptive behavior. In the case of a scientific investigation, one requires a higher number of identical successions before being. ready to accept the intuitively conceived causal relationship as an established reality, Even then, in the area of biology at Jeast, the causal relationship remains basically conjectural as long as the number of repetitions is not infinite. Nevertheless, we have to admit that, even in the pursuit of scientific interest, the number of repetitions required be- fore the impression of pure coincidence is eliminated is relatively soon reached. After all, the willingness to think post hoc, propter hoc depends to a con- siderable degree on the personality of the observer. Irrespective of the number of repetitions, the persuasive power of the repetitions depends on conditional factors. There are men, for example, with a strong inclination to associate a comparatively short series of successive similar data with one an1280 other in the sense of a causal relation- ship. On the other hand, one knows laymen and researchers of outspoken skepticism who will not integrate successive similar data into an inferred causal chain even when the probability of an accidental succession is low. In the tendency to integrate or not to integrate, the individual’s temperament, his previous experiences, his physical health, and his biological constitution play a not unimportant role. When the probable causal relationship offers a reward, he may be more likely to accept it. Further, mental age is a factor —as is seen, for example, when the child experiences a fairy tale as reality. The young,still inexperienced observer instinctively attempts to find a causal relationship, while the mature person is critical and does not exclude accidental succession so quickly. In the end, none of these arguments alters the fact that reality provides no objective criteria for arriving at a construct of causal relationships. The Physiological Basis of Consciousness The waking human being or higher animal has a large number of sense organs for making contact with the internal and external environment. The sensory cells function as receptors of organ-specific stimuli. Light flashes, for example, stimulate the rods and cones of the retina of the eye. Thereby the order of optic phenomena generated by the visual system is transformed into patterns of excitation of the visual path- ways whose morphological organization is relatively well known (9)—for example, the projection of circumscribed retinal areas to corresponding elements of the visual areas in the occipital lobes of the brain. Far less advanced is exploration of the func- tional laws of the living bra. Actually, research in this sector of physiology is only now in process of develop- ment, The school of Jung (70) and the team of Hubel and Wiesel (77) have made significant contributions, particularly with respect to visual perception. Basic information is derived from observations concerning electrical stimulation of the visual cortex in man (4). Patients subjected to such stimula- tion in an effort to localize pathological foci in that area reported visual phenomena arising with the onset of stimulation. The sense of hearing was similarly involved when the stimulating electrode was applied to a certain region of the temporal lobe. Further, it has been experimentally established that the visual and auditory sensations experienced are associated with one another in the sense of a “causal” connection on the basis of temporal coincidence or spatial contiguity. In such cases consistent relationships between brain stimulation and subjective sensations (4) are as evident as those exist- ing between natural stimuli and a determined flow of consciousness. Findings such as the foregoing raise the question, How may the “causal” relationship between excitatory patterns of the nervous system and the development of conscious perceptions come about? Before we pose this problem, we must acknowledge that it is not now possible, and may not be possible in the future, to obtain such in- formation. The subjective experience may be a direct expression of the condition of excitation of those centers which receive and integrate the sensory signals. In this case it would be only another aspect of the same process which one can objectify in the form of evoked: potentials. An alternative explanation would be that of transmission of the integrated excitation pattern to a specific system whose principal activity is one of implementing release of the contents of consciousness. However, no criteria which would al- low us to define such a process of transmission are, as yet, known. For the entire process which leads from the sensory stimulation pattern to the content of consciousness results exclu- sively in the mediating of relevant information. The process of transmission itself lies in an area into which we have no insight. Obviously reference to a reflex mechanism leads no further, so that physiology must give up the attempt to submit a comprehensive ex- planation. This is not to deny that there is a correlation between patterns of neural excitation and the release of corresponding contents of consciousness. . This situation is not unlike that existing with respect to verbal communication. The listener is unaware of the pressure changes acting upon his eardrum, and he does not perceive their transmission upon the sensory surface within the organ of Corti. Nor is he aware of the nervous impulses which are sent from the organ of Corti to the auditory centers of the brain. . SCIENCE, VOC. 158