It's the dominant function of INFJs and INTJs and plays an important role in 8 of the 16 types. How does it work?Learn more ↓
Introverted Intuition can be thought of as a mental process that traverses a deep root system of information, usually images, in search of meaning and connection. Like all personality functions, Ni is an unconscious, natural process—the Introverted Intuitive doesn't say to themself, "I will scan all known images and experiences to search for conclusions that are meaningful to me"—they just do it.
In his book Psychological Types, Carl Jung states that "the primary function of Intuition ... is to transit images, or perceptions of relations between things which could not be transmitted by the other functions or only in a very roundabout way." Whereas Extraverted Intuition (Ne) seeks meaning by scanning images and perceptions in the external world in a spatial fashion, Ni traverses an internal tree of information in a more iterative manner. Both will experience an "aha" moment upon discovery of connection, but for Ne it is usually a discovery of a possibility, and for Ni it is often the discovery of a conclusion, or a solution to a problem.
Ni is similar to Introverted Sensing (Si) in that both have vast databases of internal information that they maintain and refer to constantly, but they are interested in different things. Si wants to perceive the reality of an image, and will stop processing once it has gotten a complete picture. Ni seeks to understand the big picture story that an image points to, and must suppress Sensation to do its thing. A personality using Ni does have sensations, of course, but they are not guided by them—they are basically jumping off points for perceptions.
For example, imagine a news story about a hiker who fights off a mountain lion attack. Si will gather all facts about the situation, look at the image that is produced, and become fascinated by the physical pain and struggle that the hiker went through, which will become a stopping point for Si, having gathered a clear picture of the physical reality of the incident. Ni will conjure an image of a hiker being attacked by a mountain lion and become fascinated and arrested by it. Ni will explore every detail of it, without considering the actual physical experience. For Ni, images exist in and of themselves, without relation to a real, physical person or object, and importantly, without relation to themself.
Jung often wrote that Ni is concerned more than other personality functions with the collective unconscious. He firmly believed that the database of images and experiences that are available to Ni are largely unconscious, and include a wealth of data inherited from the collective. Assuming this to be the case, Ni can be said to have a foot in the unconscious world of today, perceiving what is going on in the world of humans at a deeper, more fundamental level than other functions, while remaining unconscious of doing so. One can argue for or against this idea, but all known religious prophets seem to have had Ni-dominant personalities.
The shadow function of Ni is Extraverted Sensing. Ni has little need or value for looking at every detail of their environment.