Pi, you are probably not going to be satisfied with this answer, but to put it simply: because that is just not how we are wired. There is a structure to the way our four functions are wired in a hierarchy, there is push and pull, balance and counter balance between functions, they are not just random functions all slapped together.
Our true functions are the 4 priorities: Dynamics, Stimulus, Worldview and compass. A compass is really more than just Ti or just Fi. It is more like Ti with a little bit of unconscious Fi mixed in, or Fi will a little bit of unconscious Ti. Because a compass cannot exist without both Fi and Ti, and the same goes for the other priorities. However, there is no need for us to randomly be able switch to Fi mode, because the compass priority is already taken up by Ti. Ti actually serves the same purpose in the psyche, only It does it with logic based criteria as opposed to values based criteria.
However, even though the priorities are compositional, it does not mean that you are actually consciously using both functions within them. For instance, in order for Ne to work it must be able to Perceive Se as well. Ne picks up patterns within an Se detail. But the reason you are using Ne and not Se is because Ne is delivering completely different information to you than straight Se would be. So in other words, the Se is there, but your mind is only registering the information gained by Ne.
I feel like this answer is inadequate, but really we actually have no reason to be able to have conscious control over the bottom four functions. We don't need to have conscious control over them to function, and on top of that, I think it would actually make more sense that we would not be able to use all eight function consciously just because 1. It is redundant, and 2. It actually makes the collective species stronger when these functions are done by different types and not all by one type.
I am getting closer to accepting your answers. Not quite there yet. I think your explanation may be quite alright. My way of looking for explanations leans far more to linear logic than to the "narrative" style you present. (That is a clumsy way of putting it but hopefully I'll be able to rectify later.) At some point I had planned to comment on:
Cognitive Functions 100:... but just have not gotten around to it. I am trying to do too many things and end up being a "Jack of all trades; master of none. Give me some time and I hope to get to it.
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To your current commentary when you speak of Ti, Fi and compass. I think you are saying for the INTP, the Ti so overwhelms the Fi there is no need to speak of it. Perhaps we can say it this way:
It is impossible to be subjective(I) and be objective(E) at the same time.
It is impossible to view breadth(N) and specifics(S) at the same time.
It is impossible to logicalize (T) and evaluate(F) at the same time.
It is impossible to observe(P) and direct(J) at the same time.
My query is a little different. Why can't an INTP who normally uses Ti, say they will abandon Ti and try out Fi? Suppose we try not to think of their motive. Let's say it is an experiment. They might feel uncomfortable and soon quit, but are they able to do it?
Can we come up with a correct example? Not sure. Try this:
An INTP wishes to define "good" precisely. He goes at it, comes up with his own definition. It seems acceptable to him in broad application. He tries it out with others. Seems to work and others go for it. This is Ti, Ne, Si, and Fe if I've got it right. Then one day he is not particularly thinking. Something others would rate as bad happens to him. Say a friend betrays him. He is not thinking but is greatly peeved. He feels this is bad, is enraged and abandons reason. He goes on an inner rampage.
Is this Fi or do we have a false INTP this could not happen to in the first place? If he is an INTP we could argue this is Fe, but I'm not thinking that way. I'm proposing this thinking INTP never covered the issue of betrayal when he was intellectualizing. He never thought of it and when it happened, it so surprised and angered him, instead of turning to his reason, he turned to feeling and decided, "To hell with everything else. I'm pissed and I'm right and I'm gonna kill."
The next day he wakes up, reasons it out, decides he was wrong and the feeling goes away. But was he Fi yesterday?
Example 2. Dante sees this hot babe, Beatrice. He decides she will lead him throughout his excursions in hell he is going to write about .... an intellectual process, but all the while thinking about what a luscious creature she is. Is this Fi?