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IndigoSensor: I think I see. In other words, you are saying that regardless of what functions we actually use to do so, everyone is going to have ways to 'stay true to oneself' (an Fi function)? I would agree with that.
The way I see it is that it has less to do with our preferences for functions and more to do with what we mean by an auxiliary function. For example,
Let's say you have Fi-dominant, and you handle the external world with Se, but you also have Ni. Since Fi is dominant, your primary inclination would be to reflect on what's important to you; however an Fi-type is an adaptive function (because it decides on the relative worth of whatever comes in). Ni, on the other hand, is a directive function b/c it has a vision of how you (or the world) should be. To take an example from the cognitive process test, Ni allows you to "Transform yourself by focusing inward on a specific way you foresee you will need to be". Now for the important part.
Se (and Ne for that matter) is also an adaptive function in that it hones in on relevant information in real-time as well as allow one to respond quickly to a change in the environment. Suddenly, however, we've just gotten to the issue that Se is actually allowing one to make rapid decisions! The reason for this is that Fi and Se (more generally, Ji and Pe) are two sides of the same coin. Se takes in information while Fi decides what to do with it, which shapes Se to change what it sees as 'relevant', and so on.
On the flip side, Ni and ...in this case Te are directive functions, meaning they have an agenda to push. Ni decides how things should be, while Te executes what should be done. Thus someone who has an Fi-dominant function will by default make use of a Pe function as auxiliary, because Ni as auxiliary cannot directly be subordinated to the function of Fi. At least, that's my interpretation; I suppose whatever Jung meant by a 'pure' type would be more clear if I knew what his response to my interpretation would be.
The tl;dr version is that the dominant function needs an opposite auxiliary to fulfill its own function.