It's the dominant function of ISTJs and ISFJs and plays an important role in 8 of the 16 types. How does it work?Learn more ↓
Introverted Sensing (Si) can be thought of as a process of merging perceptions that come from the external world with a giant internal database of everything known and experienced by the personality owner. Unlike Extraverted Sensing (Se), which has the goal of being as objective as possible, and thus takes in everything in a concrete, literal fashion, Si shades all incoming data by referencing it against its internal database of sense impressions, making its perceptions less objective to physical reality, and also more timeless and complex. Si, like Introverted Intuition (Ni), is a subjective form of perception. Like all personality functions, Si is an unconscious, natural process—the Introverted Sensor doesn't say to themself, "I will use everything ever known about this situation to determine what it really is"—they just do it.
In thinking about the Si database itself, one could liken it to a huge data warehouse containing images representing objective realities at various points in the history of the Sensor's life. Jung believes this database is augmented by the collective unconscious, and that (at very deep levels) it contains all sense impressions ever experienced by humans. It's impossible to say whether or not this is true, but we can say that the sheer number and variety of sense impressions in an Si database form an impressive record. Si operates on this data in a fashion similar to the way way some modern AI algorithms work—it doesn't reason on its own, but scans the data to find physical similarities and forms a subjective synthesis.
In his book Psychological Types, Carl Jung describes Si as having "a subjective factor, for besides the sensed object there is a sensing subject who adds his subjective disposition to the objective stimulus," and "[Si] is related primarily to the subject and only secondarily to the object." In other words, Si is more concerned with its database of sense impressions than what it encounters in the world outside, but it needs incoming data to stimulate a journey through its rich inner world of images.
Where Introverted Intuition (Ni) is concerned with meaning and possibilities in its inner images, Si is concerned with physical reality. For example, imagine a news story about a hiker who fights off a mountain lion attack. Ni will call up an image of a hiker being attacked by a mountain lion and become fascinated and arrested by it. It will then explore the meaning and implications of the image, without considering the actual physical experience as part of the image. Si will gather all facts about the situation, look at the image that is produced, compare it with similar images and ideas in its database, and become fascinated by the physical reality of the situation—such as the pain and struggle that the hiker went through. Si will stop processing when it has formed a subjective impression of what really happened.
Because its sense impressions are subjective, Si is sensitive to and dependent on a good auxilliary Judging function that will force the personality owner to express themselves. Communication is not necessary or even interesting to Si, who wants only to bask in the "real" world that lives inside, but without the ability to communicate, Introverted Sensors can become isolated, only ever residing comfortably in their own inner world, which no one except themselves understands, while losing the ability to navigate the world around them. The sense impressions of Si need to be expressed, whether through art, or writing or talking, to keep the Introverted Sensor connected to real life.
The shadow function of Si is Extraverted Intuition. Si has little need or value for seeking meaningful possibilities in the world around them.