Mind melds
A procedure known as a mind-meld involves physical contact with a subject (though instances of mind-melds without contact have been seen), making it possible to share thoughts, experiences, memories, and knowledge with another individual. Vulcans can perform mind melds with members of most other species, most notably Humans, with Jonathan Archer being the first known Human participant in such a ritual in 2154. Even the Earth Humpback Whale can be successfully melded with. The Ferengi are one of the few races known to be impervious to the mind meld; mentally disciplined Cardassians may also be resistant to mind melds if properly trained. It is un-established if this potential ability is inherent to Cardassians, or if members of any race could be trained to resist a mind meld. Machines, such as the Nomad probe, have been melded with even if only through complete contact. Though not canon, in the Star Trek: The Animated Series episode "One of Our Planets is Missing", a touch-less melding of a gaseous nebular entity was depicted.
Mind melds have been used to erase memories, as Spock performed on James T. Kirk in the TOS episode "Requiem for Methuselah". Mind melds can also allow more than one mind to experience memories and sensations, and sometimes even interact with the memories, as seen in the Star Trek: Voyager episode "Flashback".
Because it essentially bonds two minds together, thereby sharing thoughts and secrets with one another, the mind meld can be considered a very intimate form of contact, and thus not one to be taken lightly. Although mind melds are frequently portrayed as amicable (thus between two consenting beings), such is not always the case. In the TOS episode "Mirror, Mirror", Spock of the Mirror Universe performed a forced mind meld on Dr. Leonard McCoy in order to learn what McCoy was keeping secret. Mind melds can also be very violating and potentially harmful under certain circumstances. An example of this is seen in Star Trek VI: The Undiscovered Country, when Spock was required to mind meld (as a last resort) with Valeris in order to forcefully take from her information she had in order to prevent a war. The incident was visibly traumatizing for Valeris, as she tried to resist the mind meld before Spock overpowered her.
The use of the mind meld was taboo for a period of time. In the Vulcan timeline, this changed when experienced melders were shown to be able to cure Pa'nar Syndrome, a condition passed on by melders who are improperly trained and claimed to be incurable by the Vulcan government. Within a week of the Kir'Shara incident in 2154, the stigma against mind-melders was evaporating, and sufferers of Pa'nar were being cured in large numbers. By the mid-23rd century, the mind meld is a fully accepted part of Vulcan society, and was even used once to rejoin Spock's katra with his healed physical body (see below).
As originally depicted in TOS, mind-melds were considered dangerous and potentially lethal. Over the course of the original series, however, the element of risk was no longer mentioned, although it was revived on Star Trek: Enterprise with the revelation that Pa'nar Syndrome can be transmitted this way.
For a number of years, it was held that not all Vulcans are genetically capable of initiating a mind-meld, such as T'Pol. However, the overthrow of the Vulcan High Command in 2154 revealed that this is not the case, and T'Pol conducted her first mind meld soon after.
Some Vulcans appear with advanced mental abilities. For example, in the TOS episode "A Taste of Armageddon", Spock was once able induce uncertainty in the mind of a prison guard on Eminiar VII, and in the episode "The Devil in the Dark", he was able to perform a limited mind meld with a Horta without actually making physical contact with the being. It is made apparent that a touch-less meld is limited in effectiveness compared to physical melds. During more intense melds, the melder is sometimes shown using both hands.
Mind melds have at times been depicted as something from each person involved is shared. In the Star Trek: The Next Generation episode "Sarek", Jean-Luc Picard "shared" his composure with Sarek while Sarek more or less traded his emotional release at the hands of Bendii Syndrome when they melded.
