In pseudoscience literature one frequently encounters the claim that there are some people, called "sensitives" or psychics," who somehow can pick up the thoughts of others and even transmit their own thoughts to people who are not "sensitives." This direct mind-to-mind communication is sometimes claimed to be instantaneous, and independent of distance. It is also often claimed that all people -- and even domestic animals such as cats, dogs, and horses -- possess this ability to some degree, and that ordinary coincidences are in fact no ordinary, but rather mysterious demonstrations of this supposed ability. (For example, one suddenly thinks of Uncle Charlie for the first time in years, and then later learns that Uncle Charlie was in a serious accident at about the time he mysteriously sprang to mind.) All such pseudoscientific discussions of ESP or telepathy also claim that it is "proven beyond a doubt" that ESP or telepathy exists.
The "proof" quoted is usually of an anecdotal variety -- that is, a large collection of unverifiable stories like the "Uncle Charlie" story above. Sometimes, however, it is claimed that "scientific tests" at "respected" research institutions have conclusively demonstrated that ESP exists; or "government tests" have proved it; or, that the Russians are "working hard" on it; or, that the CIA uses it, etc., etc. Sometimes, but rarely, specific experiments are cited as having confirmed the existence of telepathy, clairvoyance, precognition, telekinesis, or other such "supernatural" abilities in humans or animals.
Serious questions can be raised concerning all the claims listed above. First, it is the essentially unanimous opinion of psychologists that the existence of ESP has not been shown in any experiment conducted to date. In fact, all procedurally valid and reproducible experiments have failed to demonstrate the existence of any non-sensory channel for information; in experiments where fraud and trickery are ruled out by tight controls, no "extrasensory" abilities are ever demonstrated, even by world-famous "psychics." (See the first three references below for detailed reviews of the best-known experiments.)
It is also important to realize that the existence of an ESP ability in humans or other animals would not be consistent with anything we know about nature, either from the standpoint of physics or from the standpoint of physiology. Let us consider the physiological aspect first.
All of the "higher" animals show the same fundamental organization of their sensory systems. The specialized cells (neurons) that form the central nervous system (CNS) of man and other higher animals are themselves insensitive to sensory stimuli. For each kind of important stimulus in the environment, animals have evolved highly specialized sensory organs. Each such sense organ contains unique, highly adapted cells that are sometimes called "transducers." Each stimulus in the environment involves a special kind of cellular activity. Vision involves direct detection of particles of light (photons). Hearing involves direct detection of organized, wave motion of air molecules. Smell and taste involve direct detection of molecular shapes. Sensory organs (eyes, ears, and nose) support the cells that are specialized to detect photons, molecular motion, and molecular shapes directly. These cells generate impulses that travel along nerve fibers and which are then processed in intermediate switching and coding areas, finally reaching the brain in a form that can be interpreted by brain cells.
The point is that the brain itself is insensitive to sensory information. If one opened a skull and exposed the brain to light, sound, heat, smells, etc., the brain would be totally unaware of the application of these stimuli directly to its tissues. For obvious reasons, the sensory organs containing the transducer cells are located at or near the surface of the body in all animals, including humans. When we apply this universal rule of nature concerning information input to the brain to claims for telepathy, we come up short on all accounts. Assume that some kind of "something" is radiated from a person's brain as he or she thinks. How would another person's brain ever know about it? Nowhere on the surface of the body is there a specialized organ that appears to lack a function, and which contains transducer cells sensitive to "unknown forces." Nor, contrary to popular myth, is there any large area of the brain whose function is unknown, and which might be responsible for reception and interpretation of signals from the hypothetical ESP organ.
Furthermore, in the course of evolution many kinds of animals have developed extremely acute senses of one kind or another, compared to those of humans. Dogs have much more highly developed sense of smell than do humans; hawks and eagles, more acute eyesight; bats, much wider range of hearing, etc. Where is the animal that has a much more highly developed ESP-sense than humans? In fact, the ability to sense the presence of predatory animals that could not be seen, heard, or smelled would confer such enormous advantages for its possessors that the evolution should have made ESP as common as four, claws and moist noses. It has not happened. Could it be that no such sense organ exists because there is no stimulus for the organ to detect?
Some persons argue that only human beings are capable of ESP communication; or, that only certain, special persons are so endowed. Comparative anatomy fails to show any evidence for either contention.
A proponent of ESP could argue that telepathy differs from all other senses in that the brain itself is the telepathic sense organ. In this case the detected stimulus would require the penetrating power of X-rays or nuclear radioactivity in order to get through the skull to reach the brain! This brings us to the realm of physics, where ESP falls down as badly as in the realm of physiology.