Siddhi is typically defined as "a magical or spiritual power for the control of self, others and the forces of nature." The siddhis described by occultists and yogis are in actuality supernormal perceptual states available to all human beings. These are absolutely natural abilities that can be explained in highly rational terms. There is nothing mysterious or magical about the siddhis. More formally Siddhi can be defined as follows:
SIDDHI (Sanskrit -- Accomplished One). A term for different capabilities: Through recognizing emptiness, clarity and openness of the mind, different qualities arise naturally, since they are part of mind. The Buddha, whose personal name Siddharta is based in the root-word and means "he whose aim is accomplished," distinguishes between two types:
Normal Siddhis: all those forces of the conditioned world that transform elements.
Extraordinary Siddhis: the ability to open beings up for the liberating and enlightening truths; to lead to Realization.
Just like any other natural human ability, different people display differing abilities towards learning and/or spontaneously displaying siddhis with Karma often playing a primary role. Some people are born with siddhis that they exercise without being aware that their particular psychic gift is unusual. In such cases, it may come as a traumatic event to the individual when they learn that their ability is not common and that they are considered a "misfit" by other people not possessing the psychic ability.
In other cases, one can practice yoga and actively develop siddhis. In addition to birth and Karma the Yoga Sutras of Patanjali, Chapter IV, verse 1 states the power of siddhis can come because of the use of Mantras, and/or Samadhi. According to occult theory, this is the rational and desirable way to go about achieving siddhis.
Another means to trigger siddhis also mentioned in the sutra, albeit in an unexpected and uncontrolled manner, is by the use of certain drugs. For example, certain hallucinogenic drugs an herbs such as LSD, mescaline, peyote and others. However, UNLESS used under the auspices of an experienced spiritual guide similar to Native American rituals that use Sacred Datura for example or the Mazatec Velada Ceremony they can stimulate siddhis in an uncontrolled fashion and quite possibly lead to an internal mental environment that causes great psychological trauma. See Aushadhis, the power of siddhis through herbs.
As well, a variety of other stimuli may cause siddhis spontaneously, such as a fall or a blow to the head.
No matter in what fashion the siddhis are produced, the fact is, THEY EXIST. The existence of the siddhis is doubted by most people because they have no experience with them. However, the act of dreaming is considered a siddhi. Thus, any one who has dreamed has utilized a siddhi.
Our so-called normal psychological attributes bleed imperceptibly into the perceptual realms (the Planes of Nature) opened up by the awakening of the siddhis, thus it is not clear cut at all as to where "normal" psychological behaviors end, and siddhis begin.
And to further complicate the situation, though academic psychologists see many cases of people experiencing siddhis, the academic psychology community, on the whole, is completely unaware of the nature and existence of the siddhis. Often this leads to the psychologist or psychiatrist treating individuals who are experiencing siddhis in an inappropriate and damaging fashion. Often, cases of so-called insanity or psychosis are cases of people experiencing siddhis who are scared and confused, in which case the individual may be given completely inappropriate treatment that only worsens the individual's condition. Of course there are valid forms of psychosis in which confinement or institutionalization are required. Again, however, the borderline between such cases and cases of people experiencing siddhis is ambiguous and ill defined presently.
In the future, Western academic science will be forced to accept the realities of the siddhis as their nature becomes better understood.
In conclusion, it must be strongly and thoroughly stressed that the siddhis are absolutely natural abilities latent in all humans. If one takes the time to learn and practice the correct yoga exercises, then it is inevitable that one will directly experience the awakening of their own siddhis. Again, there is nothing magical or mysterious going on here, and all claims put forth regarding the siddhis stand open to any type of test of their validity that anyone wishes to pose. However, those skeptical of the siddhis and who wish to challenge the claim to the existence of the siddhis must be prepared to recognize that the nature of the siddhis will not fit easily into biased misconceptions. One who experiences siddhis operates in a greater, more expanded psychological reality than one who does not and therefore the skeptic must be prepared to expand his or her understanding in an attempt to either prove or disprove the existence of the siddhis.