The J functions are often misunderstood. I'm glad this was brought up in this thread.
The J Function - One Single Function
The J functions are the processes by which we reason. Everyone uses all of them, regardless of type, when reasoning. The distinctions are in preferences in deduction process, not distinctions in function or capacity. To say that someone is Fe dominant is not to presume that they have no ability to use preferences associated with Te or even Fi. There is no truth to this because Fe, Te, Fi, and Ti don't actually exist as entities. The cognitive processes are T, F, i, and e. When these cognitive function elements combine, they form pairs. These pairs are not distinct sections of the mind, nor are the elements that comprise them. They are all parts of the whole that is an individual's ability to reason - half of the process of cognition.
In the case of our Fe dominant individual, it simply means that the individual prefers to base their deduction process from a perspective of F and e. This implies that their corresponding preferences are likely preferencially paired. In this case T and i. Again, these are nothing more than preferences in process, not capacity, and are certainly not exclusive. An individual with a strong ability to reason will develop their F, T, i, and e in turn, and most importantly - will be able to use them in alternate conjunctions. Our Fe and Ti individual will eventually develop capacity in Te and Fi preferences as a result of using Fe and Ti because they are using F, e, T, and i. This is why people tend to follow the pattern of Fe, Ti, Fi, and Te as preference heirarchy. Individuals find a successful method of thinking and base their reasoning upon it, diverging as required. Hence, preference.
To presume that because someone is Fe dominant, they cannot Te and take a measurement, Ti and understand how something works, or Fi and have a sincere emotion is simply foolish. When someone has a preference for Fe, it simply means that they prefer to start their reasoning with F and e. This is what causes 'personality'. One person gives more preference to Fi while another Te. Both of them have the ability to use the other functions, they simply prefer their more familiar functions.
Okay, now that we have that misconception beaten down...
Thinking and Feeling - Cognitive Reasoning
F is called Feeling. There is a tremendous misconception about this function because of the name. The assumptions center around this function being based in emotion. It isn't. Emotions are not cognitive functions. Emotions cannot reason. The F function is not 'emotional logic'. The F function is philosophical reasoning. It is concerned with emotion, but it is also concerned with relationships, interactions, proportions, associations, and the abstract. The F function is non-verbal right brained reasoning. At it's core, the F function asks 'why?'. The F function is responsible for assessing needs, and making inferences that have no measurable quantification, such as non-verbal communication. How the color of that shirt makes her eyes look, how the light in this room changes them both, and how it makes you feel (aka how you choose to feel) are all F functions. A lot of actual Philosophy is tied into the F function, but don't confuse the subject of Philosophy with philosophical reasoning.
When F is extroverted, it becomes focused on the philosophical relationships, interactions, proportions, and associations of the shared world. When F is introverted, it becomes focused on the relationships, interactions, proportions, and associations of the individual. It is impossible to be F dominant and not have a capacity for both of these approaches. The difference in personality lies in which is the individual's preferred function. Fe dominants are more inclined to look at the relationships in the world around them as distinct and seperate parts of a whole in which they are included. Fi dominants are more inclined to take a perspective of their own feelings and how they are affected by the relationships in the world around them. The distinction is subtle because the F function is inherently both Fe and Fi. Fe's will tend to be sympathetic, while Fi's will be empathetic. Fe's will tend to prioritize the needs of others as equal or ahead of their own. Fi's will tend to put their own needs as a higher priority than those of others, or at least equal.
The T function works exactly the same way. It is inherently an entity unto itself. It is pragmatic, logical, and left brained reasoning. The T function inherent asks 'How?'. When the T function is extroverted, it is generalized, and applied to the world outside the individual as a whole. When the T function is introverted, it is applied to the specific, and the workings of a particular object or subject. Te preference people tend to focus their logic on the workings of the big picture. Ti preference people tend to focus their logic on the workings of the individual parts.
However, F cannot exist without T and T cannot exist without F, because they are both parts of the overall J function. T has to have philosophical reasoning, or it could not grasp anything that is not perfectly logical. F has to have logical reasoning, or it would be purely reactive unassociated instinct. Both T and F comprise the J function as a whole. To say that someone is Fe dominant implies that they are also using the Ti, Te, and of course Fi inclinations of their ability to reason - at the same time. They just tend to lean more on the Fe side of their J function. Again - preference.
Strong Functions Do Not Denote Preference
Regardless of which of these function pairs is an individual's most comfortable, and base reasoning method, everyone has all of them at their disposal to a degree of capacity based on that individual. Someone may even have more capacity with a function pair that is not their dominant pair. Take Satya for example. His first step in reasoning is philosophical deduction, but he has amazing logical abilities which he uses to validate his philosophical reasoning. He might have a more effective T than F, but that isn't where his reasoning begins, nor is it the basis of his thought process. He begins, bases, and ends his reasoning with an Fe-centric cognition. This is where his personality as an INFJ comes through, and is the basis for the MBTI, which is designed around the notion that we have cognitive preferences. Capacity and function with cognitive processes are almost irrelevent to personality.
Preferences Do Not Denote Individuals
The biggest misconception is that the MBTI types are in any way indicative of an individual's personality, preferences, or other functions in life. Because of the way that the J function works, as a whole entity, but with a lean in one direction (even if that lean is toward balance), the only inference that can be derived is how and why a person comes to the conclusions that they come to. Fe dominants are going to base their thinking in F and e. They'll dip into T and i when detailing. They'll apply T to their F and e. They'll apply F to their T and i. But, they'll lean toward Fe modes of thinking. That's all. One Fe person might feel very passionate about football and yell at their television, while another feels strongly about the peace and tranquility that is violated. Another Fe person might feel passionate about politics or an election, while another feels that politics are a blight on society and should be removed. Another Fe person might feel a great urge to help the homeless, while yet another sees them as a problem and feels a great urge to run them out of town. What they all share is simply a philosophical reasoning applied to the world around them, whatever it is that they are concerned with.
Aw, shucks.
