p. 41: "When we use Feeling in an Introverted way, it operates as a kind of inner flame--a sense of personal values that may be difficult to explain or express directly but whose character informs our choices and inclinations."
p. 366: "Introverted Feeling ... encourages a personal relationship to an evolving pattern, a will to gauge the situation by an experiential ideal. For example, if we use Introverted Feeling to make a good spaghetti sauce, we won't follow recipes or measure ingredients. We'll sample the sauce as we're making it, gauging its taste, smell, and texture by their ideal outcome and adjusting for circumstantial variables so the emerging pattern stays on track."
p. 367: "To invoke Introverted Feeling, we have to know the difference between a good outcome and a bad one--know with our senses, in our bones [on the basis of living, breathing, first-hand experience]."
p. 370: "Introverted Feeling relies on the inward, right-brain criteria of experience and empathy to mark off decisions that go beyond our roles in society to affect us as human beings. Law and custom, after all, are the lowest common denominator of a defined community. We associate character and humane behavior with the moral imperatives shaped by inner values."
p. 371: "An inner point of reference, one trained by personal experience. [Bypassing matters of social standing] to focus on the quintessentially human."
Proposed definition #1
Introverted Feeling (Fi) is the attitude that everything that is manifest (apparent, observable, described) is the expression of a soul or life force, in terms of which everything ultimately makes sense. Everything that happens is the result of a soul expressing its unique nature.
From this attitude, each living thing is completely unique, and has unique needs. Every living thing needs to express itself and grow in its unique way. None of this can be put into categories or measurements, at least not without blotting out that utter uniqueness of each living thing. Because we are all living things, even though each of us is unique we can still connect to the life force as it exists in others. From an Fi standpoint, the way to respond to things is in a way that is faithful to that underlying life force.
Proposed definition #2
Introverted Feeling (Fi) is the attitude of judging things good or bad based on how they harmonize or clash with a living being's inner essence. That inner essence or soul, and how things in the environment get along with it or conflict with it, is knowable only first-hand--ultimately, only by that soul. It is known by attending to one's own emotions in response to things. What you like is good--for you, not necessarily good for others. What you don't like is bad--for you, not necessarily bad for others. Anything outside your own soul is irrelevant to evaluating anything or choosing your course in life.
As a language of Ego Orientation
As a Dominant Function, Fi leads IFPs to live a life based on empathy and harmony between self and others--and/or to see life as a never-ending conflict between souls that are intrinsically different and opposed. ISFPs typically seek out a space in which they can be completely and spontaneously themselves, following their artistic impulses without regard to social expectation or definition of any sort. Some do their best to live life as a soap opera: creating and living out intense drama wherever they go. INFPs typically seek to understand the world in terms of drama, emotion, and people seeking their own unique callings (perhaps Garrison Keillor is a good example of that). Some, like John Gray, attempt to help others understand each other through empathy with each other's differences, and thereby find peace and synergy.
Developed Fi naturally leads people to favor mercy or forgiveness for people who have done heinous acts--anything from theft to murder to genocide--acts that, under the ordinary laws that make a society manageable (see Extraverted Thinking), would usually merit their imprisonment or execution. From a developed Fi perspective, the criminal is still a living soul, still unique and precious despite whatever he may have done. If we walked in his moccasins for a while, maybe we could see it his way. Without condoning his crimes, maybe we could see how we ourselves could have done the same things under similar circumstances. This use of empathy as one's ultimate anchor of orientation leads to a resolute non-judgementalness. First empathize--find something in your own heart that lets you see how someone could feel and act the way he did--and then you will probably find that you no longer feel hatred or a desire for retribution.
As a Secondary Function, Fi typically leads EFPs to tune into the unmet needs and callings of others--as an avenue to making a sale, as a way to intuit what would entertain people, as a channel to political gain by demonstrating that you understand people's pain (e.g. Bill Clinton), as a way to chart a course through life based on a calling felt to be unique to them. Sometimes it leads them to sense a higher calling to answer to, a sense that their actions have cosmic meaning by virtue of how they aid or hinder life.
As a Tertiary Function, Fi typically leads ITJs to retreat into solitary actions that have no constructive worldly effect but are aimed at providing a justification for calling themselves good people. Another example is obsession with the purity of one's soul. For example, being a vegetarian while working at Taco Bell--not out of any great love for animals (the person might hardly know anything about what cows are like), but to be able to say, "Well, at least I never ate any animals." Or engaging in pointless acts of honor, like maintaining super-self-control or "doing one's duty" or going down with the ship. Nothing is gained by going down with the ship; it's a hyper-introverted act aimed at providing a rationalization for one's goodness without regard to real-world consequences. Nearly all of these tertiary-Fi acts involve refraining from action viewed as unethical rather than taking positive action that would accomplish something. They're a retreat from the world--or rather, a rationalization for disregarding worldly matters.
As an Inferior Function, Fi typically leads ETJs to acts of self-destructive hedonism, creation of opera-like drama in their lives and the lives of those around them, obsession with "integrity" (like going down with the ship), instant and irresponsible abandonment of anything they don't like (the opposite of going down with the ship), and bizarre solitary acts of atonement for the harms they've done to others. Sometimes inferior-Fi leads ETJs to preach and even practice a sort of hyper-selfishness, e.g. Ayn Rand and the Landmark Forum. "I'm doing fine, so why should I give a damn about you?" (Very different from highly developed Fi, which leads you to see all people as connected and the highest joy of life as the experience of that connection.)
Tertiary and inferior Fi also sometimes lead TJs to view large numbers of people as "troglodytes": soulless or stupid creatures whose rotten situations in life derive only from their own intrinsic rottenness-of-soul. To take a comic example, Lex Luthor's lamentation in Superman, "Why is the world's greatest criminal genius surrounded by nincompoops?"
Perhaps the most typical manifestation of tertiary and inferior Fi is an attitude of psychologizing other people: a sort of pseudo-empathy in which one explains other people's behavior in terms of pitiful needs and psychological flaws that anyone would be ashamed to have. "Notice the defensiveness. He clings desperately to his ideas. Such weakness." (Nearly all psychological theories offer plenty of ammo for psychologizing, including Lenore Thomson's ideas.) Where developed Fi leads you to find something in your own soul in terms of which to truly understand someone else and see things their way, tertiary and inferior Fi typically lead you to find something in your own soul that you despise, in terms of which you can "explain" them and justify putting them down.
Naturally, you can see plenty of dominant-style Fi in ETJs, secondary-style Fi in IFPs, and so on--even inferior-style Fi in IFPs.
Introverted feeling is judgement with an emotional slant that causes the individual to view the object on a Subjective level. It is primarily a silent inaccessible function that is difficult to conceptualize. Therefore, unlike its extraverted counterpart, Extraverted Feeling, it is entirely individualistic, with a leaning toward the mystical. Introverted feeling is generally disconnected with typical external stimuli. Introverted feeling is only concerned with the external to the extent that the object has some relevance to a deep, internal value. Its primary objective is to harmonize ideologies, concepts, relationships etc. with the internal guiding force.
Whatever the individual values the most will dominate the motivations, goals and chief objectives of the individual. For example, if the primary value is God, then all other values will find themselves inexorably subjugated to this primary one. Often, unbalanced introverted feeling will create in the individual dramatic mood swings and decisions based on illogical rationales. However, at its best, introverted feeling provides a navigational quality that creates in the personality tenacity, idealism, honor, relationship wisdom and a unfaltering value system that is seldom compromised.
Introverted feeling is a counterpart to Extroverted thinking, as are all inferior elements to the dominant. Everyone attempts to accomplish the inferior through the dominant. By following first principles and proverbial logic (Te) i.e. literal proverbs, they accomplish being "good"(Fi). Though the elements look different they simply reverse the horse and carriage. An Fi will look at a business tycoon and blame them for the state of the poor. A Te realizes that simply feeling bad for the poor won't accomplish anything, because you can't take emotions to the bank (a common Te proverb) you need to actually DO something. Even if you don't FEEL for them, building a huge corporation and then donating your equity to charity is worth more than a single person volunteering their entire life. For this reason, even though still hated by a generally large amount of people, Bill Gates (textbook ENTJ according to typology forums) is more effective in donating ~$30 billion to build business around the globe for the poor and promoting philanthro-capitalism than an army of Mother Teresa's.