Extra Sensory Perception (ESP) is more popularly known as the sixth sense because it enables the possessor of this power the ability to sense things and events that are not perceptible to the regular five senses of sight, sound, smell, taste, and touch. Since this ability is outside normal range, it is also accepted as being independent of time and thus enables the witnessing of unknown events from the past and prediction of the future. ESP was first subjected to close scrutiny in 1882, the year in which the Society for Psychical Research was founded. Once the journals of this society became published, similar observations were reported from other parts of the world.
The initial studies of ESP were not really experimental in nature and hence their results are often disputed. Most of these studies concerned themselves with spontaneous events that might have had simpler explanations. Several of the people that were studied in these experiments claimed to be psychics and such though there was no way of actually establishing that detail. They were accepted on the basis of their claims. The whole "experiment" was more like a courtroom brawl with the researcher behaving like prosecuting attorneys who were questioning the accused. The witness who handled this examination the best was deemed creditable.
Though the number of people who claim to have ESP is very small, the truth is that nearly all human beings have episodes when their minds function at a heightened efficiency and make leaps in reason that are often termed as intuition or hunch. Everyone is aware today that human beings do not use their brains to its full potential. We are always working on a fraction of what our brains are actually capable of. Whatever the psychological and physiological reasons for this, it remains a fact that it is possible for the brain to have occasional leaps in efficiency.
There are certain chemicals that can increase the input-output rate of the human brain though they cannot be used on a daily basis because the extra work heats up the brain like an engine working faster than it can be cooled through conventional means. The natural cooling mechanism of our brain can only do so much. This has been a frequent topic in science fiction with many kinds of "coolants" being used to keep the brain cool while making it work at a much fast pace to achieve ESP like powers. Similarly, some concepts deal with ignoring the body and consuming only such things as are required by the brain for optimal functioning.
The stomach is the biggest competitor of the brain in terms of energy requirements. During digestion, the brain gets sluggish and operates at a much lower efficiency because the stomach increases its energy consumption to complete the task of digestion. What few people know is that the stomach also has neurons, the same things that make the brain "think". Perhaps this is the origin of the term gut instinct.