The Myers-Briggs posits four Judging functions and four Perceiving functions. The Perceiving functions are further subdivided into two Sensing and two Intuition functions. One Sensing function, Extraverted Sensing, is directed outwardly, while the other, Introverted Sensing (Si), is directed inwardly. Se serves as the dominant function for ESTPs and ESFPs, while Si is dominant in ISTJs and ISFJs.
Extraverted Sensing (Se) & Introverted Sensing (Si)
Extraverted Sensing (or what Jung called Extraverted Sensation) occurs by way of the five primary senses (sight, sound, touch, smell, and taste). Introverted Sensing (Si), by contrast, relates to inner bodily sensations such as pain, hunger, thirst, internal temperature, numbness, tingling, muscle tension, etc. Both Se and Si are critical for our physical survival, delivering vital sensory feedback from within and without.
At first glance, it may seem strange that some people would display stronger extraverted senses and some stronger introverted senses, but this is really no different than other individual differences. There have surely been times in our evolutionary past in which a well-developed Se was more beneficial to survival, as well as times where Si proved especially important. Consequently, having both Si and Se types would have improved the odds of our survival as a species, allowing us to receive and monitor a broad swath of sensory data. Se types, for instance, were likely better suited for hunting tasks because of their ability to notice and respond to important details in the environment. Si types, by contrast, may have been better at discerning whether food or water was poisonous because of their inner sensitivity.
Despite the important role of Extroverted and Introverted Sensing in mediating basic physiological perception, this is not their sole purpose. More broadly, Se can be understood to involve a concern for the concrete events happening around us. This not only includes noticing sensory details such as sights, smells, and movements, but also things like trends, fashions, and styles. While all personality types rely on vision for everyday functioning, Se types seem especially attuned to visual input. This is why they (SPs) tend to be more concerned about their appearance, as well as appearances in general, than Si types (SJs) are. SPs seek pleasure and Se takes great pleasure in perceiving both physical beauty and sensory novelty. Their penchant for sensory novelty is why SPs are commonly described as thrill-seekers or hedonists. Se is also engaged by physical action. SPs love perceiving and physically responding to environmental cues. This why they often take up work as first responders, athletes, mechanics, chefs, and the like.
While Introverted Sensing can attune to immediate inner sensations (a role that, by the way, is commonly overlooked), it is also associated with remembering and preserving past ways of doing things. I like to think of Si as a synopsis of one’s personal past, in which all of one’s past experiences are condensed into a specific view of the past. For Si types (SJs), in particular, the things that are most prominent and cherished in this Si perspective are those which are most routine and familiar. There seems to be a quantitative factor at work in Si. The more times something is done—eating a certain meal, hearing a specific song, etc.—the more preferable it becomes. It was probably an Si type who, in noticing how his tastes changed with repeated exposures, coined the phrase “it will grow on me.” In many cases, if you can get an SJ to keep trying something, there’s a good chance they will come to enjoy it (or at least better tolerate it). SP types, who are less attached to past experiences and generally seem to have a broader palate, are more apt to like something the first time around. SJs, by contrast, prefer that extraverted sensations remain within a familiar bandwidth. Novel or extreme external sensations can even seem intrusive to SJs (especially ISJs), causing them to switch from Si to their less preferred Se function. SPs may feel similarly put off when admonished to “listen to your body” or “notice when you are full,” as doing so would require a greater focus on Si than Se.
As explored in my post, Introverted vs. Extraverted Functions, the introverted functions can generally be considered more conservative and the extraverted functions more liberal or expansive. This holds true for Si and Se. Introverted Sensing types are conservative with respect to the past. They prefer the “tried and true.” And since they don’t rely on novel sensory input from the environment to stimulate them, they tend to be less consumeristic or materialistic than Extraverted Sensors might be. When Si types are in need of something new, they often prefer to use Ne to cleverly fashion a makeshift solution from existing resources rather than running out to buy something premade. In this sense, Si (or Ne) types are often described as resourceful. Se and Ni types, by contrast, would be more apt to go out and buy whatever they think they need without giving it a second thought. Their first instinct toward the material world is not to conserve, but to consume. This is especially true of ESP types.
To learn more about Si, Se, and the other personality functions, I encourage you to explore my recent book, My True Type: Clarifying Your Personality Type, Preferences & Functions: