Performing art in which practitioners appear to demonstrate exceptional mental abilities
Mentalism is a performing art in which its practitioners, known as mentalists, appear to demonstrate highly developed mental or intuitive abilities. Mentalists perform a theatrical act that includes special effects that may appear to employ psychic or supernatural forces but that is actually achieved by "ordinary conjuring means", natural human abilities (i.e. reading body language, refined intuition, subliminal communication, emotional intelligence), and an in-depth understanding of key principles from human psychology or other behavioral sciences. Performances may appear to include hypnosis, telepathy, clairvoyance, divination, precognition, psychokinesis, mediumship, mind control, memory feats, deduction, and rapid mathematics. 
Mentalism is commonly classified as a subcategory of magic and, when performed by a stage magician, may also be referred to as "mental magic". However, many professional mentalists today may generally distinguish themselves from magicians, insisting that their art form leverages a distinct skillset. Instead of doing "magic tricks", mentalists argue that they produce psychological experiences for the mind and imagination, and expand reality with explorations of psychology, suggestion, and influence. Mentalists are also often considered psychic entertainers, although that category also contains non-mentalist performers such as psychic readers and bizarrists.
Some well-known magicians, such as Penn &amp; Teller, and James Randi, argue that a key differentiation between a mentalist and someone who purports to be an actual psychic is that the former is open about being a skilled artist or entertainer who accomplishes their feats through practice, study, and natural means, while the latter may claim to actually possess genuine supernatural, psychic, or extrasensory powers and, thus, operates unethically.
Renowned mentalist Joseph Dunninger, who also worked to debunk fraudulent mediums, captured this key sentiment when he explained his impressive abilities in the following way: "Any child of ten could do this – with forty years of experience." Like any performing art, mentalism requires years of dedication, extensive study, practice, and skill to perform well.
Background.
Much of what modern mentalists perform in their acts can be traced back directly to "tests" of supernatural power that were carried out by mediums, spiritualists, and psychics in the 19th century. However, the history of mentalism goes back even further. Accounts of seers and oracles can be found in the Old Testament of the Bible and in works about ancient Greece. Paracelsus reiterated the theme, so reminiscent of the ancient Greeks, that three principias were incorporated into humanity: the spiritual, the physical, and mentalistic phenomena. The mentalist act generally cited as one of the earliest on record in the modern era was performed by diplomat and pioneering sleight-of-hand magician Girolamo Scotto in 1572. The performance of mentalism may utilize conjuring principles including sleights, feints, misdirection, and other skills of street or stage magic. Nonetheless, modern mentalists also now increasingly incorporate insights from human psychology and behavioral sciences to produce unexplainable experiences and effects for their audiences. Changing with the times, some mentalists incorporate a smartphone into their routine.
Techniques.
Principle: sleight of hand and other traditional magicians' techniques.
Mentalists typically seek to explain their effects as manifestations of psychology, hypnosis, an ability to influence by subtle verbal cues, an acute sensitivity to body language, etc. These are all genuine phenomena, but they are not sufficiently reliable or impressive to form the basis of a mentalism performance. These are in fact fake explanations - part of the mentalist's misdirection - and the true method being employed is classic magicians' trickery. 
Written "billet".
A characteristic feature of "mind-reading" by a mentalist is that the spectator must write the thought down. Various justifications are given for this - in order to enable the spectator to focus on the thought, or in order to show it to other audience members etc. - but the real reason is to enable the mentalist then secretly to access the written-down information. There are various techniques which the mentalist can use. A classic method is the "centre tear". The spectator is asked to commit her thought to writing on a small piece of paper (referred to by mentalists as a "billet"). She is told to write the thought in the centre of the billet, and sometimes a circle or a line will be added by the mentalist onto the billet to make sure she writes in the middle. She is then instructed to fold the billet up so that the writing cannot be seen by the mentalist. The mentalist then takes the billet and tears it into small pieces, which he may then burn or throw away or return to the spectator's hand "for safe-keeping". Secretly, during the tearing process, the mentalist tears out and secretes the centre part of the billet which bears the written thought, and later finds an opportunity to read it covertly. Alternatively the mentalist may covertly peek at the written thought. There are a large number of detailed choreographies used by mentalists to achieve a peek. One popular version - known as the "acidus novus" peek - requires the spectator to write her thought on the bottom right-hand corner of the billet. Typically the mentalist will fill up the other 3 quadrants of the billet with writing so that only the bottom right-hand quadrant is left clear. Once the thought is written and the billet folded, the mentalist will hold the billet up to the light to demonstrate that no writing can be seen through the paper. In the course of this action he is able, unobserved by the audience, to slip his thumb between the folds of the billet and expose a view of the bottom right-hand quadrant. He then gestures with the billet, bringing it at eye-level across his field of vision, and in so-doing is able secretly to peek at the spectator's written thought. This is only one of a number of such peek choreographies. Some involve placing the billet in a gimmicked wallet, which allows the mentalist covertly to see the writing. Others employ sleights of hand derived from card or coin magic. 
Modern gimmicks.
In addition to these traditional magicians' techniques, there is today a huge range of electronic, computer and other gimmicks available to the mentalist. These include dice which secretly transmit the numbers thrown, decks of cards which secretly transmit the cards chosen, notepads which secretly transmit what has been written etc. Smartphones have added an additional range of possibilities. For example, the mentalist can use concealed NFC tags to covertly download onto a spectator’s phone a fake version of a popular website such as Google Images, which allows him to know an image which the spectator believes has been chosen secretly.
Nail writing and its technological equivalents.
Where a mind-reading performance does not involve the spectator writing the secret thought down, generally the method employed is that the mentalist purports to predict the secret thought by (apparently) writing an unseen prediction, often behind a clipboard or other hard surface, then he asks the spectator to reveal the thought, and the mentalist at that point quickly and covertly writes or completes his prediction using a nail writer or swami gimmick. These are small devices which allow the mentalist to write unseen with his thumb under cover of a clipboard or in his pocket. 
Again, these traditional magicians’ devices have now been supplemented by technology. The mentalist can now buy blackboards and whiteboards which are capable of writing (apparently) handwritten messages fed to them remotely, small printers which can print a spectator’s chosen number and feed the printed paper into an (apparently) sealed jar, and a host of other technological gadgets which make it appear that the mentalist predicted the spectator’s thought, when in fact he simply waited for it to be disclosed by the spectator and then created the evidence of his “prediction”.
Pre-show work.
Many mentalism effects rely on pre-show work. This involves the mentalist or his assistant interacting with certain members of the audience before the performance begins. This can be in a pre-show reception, or in the auditorium itself as the audience take their seats, or even in the queue outside the performance venue before the performance. Pre-show work can take a number of forms. One type involves the mentalist talking to a spectator whom he will later, during his performance, involve in one of his effects. In this case the mentalist sets up the trick by covertly obtaining information from the spectator which he will later reveal during the performance. The interaction with the spectator may be made to seem like a casual “meet the audience” conversation, with no warning that the spectator is later to be involved in the performance. Alternatively, the mentalist may tell the spectator that he intends to involve her in his show. In that case the pre-show interaction is usually characterised as preparation “to save time during the show” or similar. Either way, the mentalist will use the occasion to obtain information from the spectator covertly for later revelation, either by traditional sleight of hand methods such as a billet peek, or by using electronic gimmickry such as a Parapad. Alternatively, the mentalist may ask the spectator to make a choice (eg a number, a playing card, a selection from a list of items) and to recall that choice when later asked to participate during the performance. Typically the spectator will believe she has a free choice, but in fact it will be a choice forced by the mentalist, This can be done in a number of ways. One popular method is a proprietary device called a Svenpad. This is a notepad in which every second page is imperceptibly shorter. The long pages each have written on them an item from a list of choices (eg film stars, holiday destinations etc), but the short pages each have the same item - the force choice - written on them. When riffled from front to back only the long pages are visible, showing the full range of different choices. But when riffled from back to front only the short pages are visible, each bearing the force selection. Other forcing methods include trick decks of cards, where all the cards are the same. During the performance itself, when the mentalist involves the spectator in his effect, he will usually aim by careful use of language to avoid any mention of the pre-show interaction to the wider audience, either by himself or the chosen spectator. His aim is to suggest that he and the spectator have not previously met. When the spectator’s covertly obtained information or forced choice is revealed, this greatly enhances the effect from the point of view of the wider audience. It will usually seem that the mentalist has elicited a wholly uncommunicated thought from a random audience member. The chosen spectator herself, having participated in the pre-show encounter, perceives a different and less spectacular effect. This is an example of dual reality (discussed below) in mentalism.
Suggestion.
This technique involves implanting an idea, thought, or impression in the mind of the spectator or participant. The mentalist does this by using subtle verbal cues, gestures, body language, and sometimes visual aids to influence their thoughts. For instance, asking someone to "think of any card in a normal deck" automatically plants the general idea of a playing card in their mind. Similarly, asking them to "visualize the card clearly in your mind" can put the image of a particular card in their imagination. 
Misdirection.
Also known as diversion, this technique aims to divert the audience's attention away from the secret method or process behind a mentalism effect. Magicians and mentalists frequently use grand gestures, animated movement, music, and chatter to distract attention from a sneaky maneuver that sets up the trick. For example, a mentalist may engage in lively conversation while secretly writing something on his palm. Or he may dramatically throw his jacket on a chair to cover up a hidden assistant in the audience.
Cold reading.
This technique involves making calculated guesses and drawing logical conclusions about a person by carefully observing their appearance, responses, mannerisms, vocal tones, and other unconscious reactions. Mentalists leverage these cues along with high probability assumptions about human nature to come up with surprisingly accurate character insights and details about someone. They can then present this as if they magically knew the information through psychic powers. 
Hot reading.
Hot reading refers to the practice of gathering background information about the audience or participants before doing a mentalism