The metabolic principle states that any idea or thought can be arrived at through the combined use of all energetic quadrants (Je/Ji/Pe/Pi) and four attitudes (T/F/N/S) when used in an individual, whether they're coming from Alpha/Beta/Gamma/Delta. Each quadrant has all faculties available for the creation of any point of view. There is no thought, idea or action that is absolutely unavailable or inexpressible in any person, but patterns do exist as well as probabilities of certain ideas manifesting more commonly across certain functions than others.
Every person has all four of their functions active and working in the psyche at the millisecond scale, whether consciously or unconsciously. Metabolism is what creates our unbroken awareness, but it happens so rapidly that we cannot see the joint effect of the functions coming together. To give an example, a Delta type will have their Te and Fi functions both active at all times, cycling hundreds of times per minute. When we consider this aspect of metabolism, it becomes very easy to see how the combined microscopic effect of Te+Fi cycling may appear to suggest "Fe" at the macroscopic level such as by the registration of a social context.
When this happens, what is actually being simulated is not "Fe" but "F+E" or an "FE" temperament. Fi is pinging its emotional register against each party involved one at a time, and coming to form a mental picture of the situation by the inner state of each person. Te then comes to understand the optimal dynamic rearrangement of objects (people) towards a desired outcome, having in mind an Fi-centric agenda. If executed effectively, the net result may be the graceful orchestration of a social context, while never having possessed the Fe function or experienced its phenomenology. What it means to have "Fe" is a very specific thing, and involves much more than the agreeableness provided by a light F attitude played out within an external setting (i.e. F+E).
To give another example, the cognitive function "Ni" is much more than I+N in CT theory, and is defined as:
Ni: Reactive, Narrative-Driven, Episodic, Predictive, Stability-Seeking, Extrapolative, Aphoristic, Thematic, Karmic Perception
The attitudes of "I" and "N" are negligible aspects compared to all the properties that really build up the function's metabolism. Ni is not intuition going inward, but is first and foremost a Pi process, which exists as a part of the Ni/Se duality with a multitude of emergent properties. The rest of these metabolic properties are not reproduced by the joint effect of two adjacent functions (i.e. Ne+Fi) simulating the I+N temperament.
Any single thought that might emerge from an actual Ni user may also emerge from a non-Ni user through the joint effort of more than one function. For example, suppose we have a TiNe who uses their Ne to create short-range hypotheses that do not settle into a longstanding thematic worldview. However, through a personal aspiration towards a unified truth, hypotheses are generated (Ne) and eliminated (Ti) until something remains that endures both the scrutiny of reason and the trials of time (Si). The TiNe can then come to deductions that are compelling in their thematic wholeness and superficially match the content of some Ni users, even if they were generated from an entirely different metabolic route through the use of Ti+Ne+Si. We must therefore be wary to leap too quickly from a specific idea or content-matter to the assumption of a function's presence as a static feature of a person's biology.
How then can we rightly identify metabolism? First it's important to remember that metabolism, while being a more fundamental aspect of Type, is unavailability for direct examination and requires the assistance of an instrument that can identify it's presence. This instrument comes in the form of vultology, which reads the effects of metabolism at a second-by-second level as the processing of the brain produces secondary effects on the face and body. Vultology is arguably a better pace to begin one's journey to identifying a person's type.
However, it is not impossible to identify type purely through the analysis of text and speech, and indeed this layer (of the four layers) acts as a necessary point of triangulation in the CT model. But before we attempt to type a person through text, we must:
There is a basic human experience we all exist within that must not be mistaken for anything type-related. And while these may vary from person to person, it's generally a good rule of thumb to assume that:
Depending on the individual, and especially due to variations in development levels and ego association, a person with a function in a tertiary/polar position can have greater talent in wielding it than someone who has it as their dominant. Type is not determined by level of success or achievement; it is not "how much" one uses a function, but "how" one processes it.
We must bear in mind that there are mediocre and brilliant versions of all the 16 types in this world. An effective social communicator does not imply Fe, a logically consistent argument does not imply Ti, and a keen prediction of the future does not imply Ni. Instead, to identify a function's true role, the focus needs to be on energetic toll, and the relationship the function has within the hierarchy of the person themselves. See the development levels article here for more information.
Lastly, we must always read the individual as a whole rather than as parts. Don't look at how any one part is operating in isolation, but try to understand the person simultaneously from every dimension of that type. If they appear to be FiNe, then that can be triangulated against Delta qualia, Delta values/approaches, Ji energetics, reviser energy, and many other aspects. If the variables don't add up from multiple dimensions, remain skeptical and open-minded. Learn more about them until their entire story makes sense.