To every theory, there are atypical members whom are often acknowledged as the exceptions.
#1. Defining judging and perceiving
Judging [Rational]: Interpreting the data
Extroverted: (Te, Fe)
Introverted: (Ti, Fi)
- Rationals; Dominant Judging (Ti, Te, Fi, Fe): IPs & EJs
Perceiving [Irrational]: Receiving the data
Extroverted: (Ne, Se)
Introverted: (Ni, Si)
- Irrationals; Dominant Perceiving (Si, Se, Ni, Ne): IJs & EPs
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#2. The P and J letters determine which dom/aux functions are extroverted or introverted
---P:
Perceives externally (Ne, Se) and judges internally (Ti, Fi)
---J:
Judges externally (Te, Fe) and perceives internally (Ni, Si)
- 'Perceivers': IPs & EPs
- 'Judgers': IJs & EJs
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#3. The E and I letters determine which dominant functions are extroverted or introverted
- Extroverts: EJs (Te/Fe) & EPs (Ne/Se)
- Introverts: IPs (Ti/Fi) & IJs (Ni/Si)
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So... # 1, 2 and 3:
'Perceivers'
I--P: Dominant Introverted Judging (Ti, Fi) and Auxiliary Extroverted Perceiving (Ne, Se)
E--P: Dominant Extroverted Perceiving (Ne, Se) and Auxiliary Introverted Judging (Ti, Fi)
'Judgers'
I--J: Dominant Introverted Perceiving (Ni, Si) and Auxiliary Extroverted Judging (Te, Fe)
E--J: Dominant Extroverted Judging (Te, Fe) and Auxiliary Introverted Perceiving (Ni, Si)
The ISFP 'theoretically' cannot use Si as their dominant preference because it is a function the SJs use as their dominant or auxiliary. It would make them a ISTJ [Si, Te, Fi], if that were the case. Perceivers initially rely on the external environment to obtain data (Ne & Se).
Does this help?