Despite being only one letter different from one another in the dichotomous system of personality, INFJs and INFPs actually share zero cognitive functions in common.
The INFJ leads with introverted intuition—a function that seeks to narrow down and pinpoint the most optimal of all possible options for the future, before considering the emotional implications of that decision. The INFP, on the other hand, leads with introverted feeling, which seeks to determine how the INFP feels about various different outcomes before looking at the course of action he or she should take to achieve the desired outcome.
The INFJ backs up their introverted intuition with extroverted feeling (or Fe). Fe reaches decisions by seeking out the emotional input of others and considering how their decision would impact the lives of those around them. While the INFP certainly takes the feelings of their loved ones into heavy consideration, their main decision making process happens internally and is guided by the INFP’s personal moral compass, rather than the direct influence of those around them.
Once the INFP has decided how he or she feels about a situation, they will envision how their choice could play out in a fanciful, long-term fashion. They may choose to pursue a particular person and envision their courtship and eventual marriage, or decide on a job and envision themselves rising to the top of the field and accepting an award for their endeavors. This is the INFP’s extroverted intuition at play—it enjoys dreaming up scenarios for the future that are both exciting and wildly ambitious.
When the INFJ thinks toward the future, he or she envisions a more realistic chain of events. The INFP’s visions of the future are ambitious and indulgent but the INFJ’s vision of the future is more pragmatic and cautious. They may wish to pursue a person or accept a particular job offer, but envision a likely series of events that would lead to the eventual downfall of either opportunity, and therefore choose not to pursue it at all.
Whereas the INFP takes more of an “If you can dream it, you can achieve it” approach to the future—which often turns out to be a self-fulfilling prophecy— the INFJ is more cautious and skeptical about the future. And while the INFP must first feel personally connected to and in tune with whichever decision he or she makes, the INFJ is more interested in determining the most realistically viable course of action and checking in with his or her feelings on it after the fact.
In practice, there are an infinite number of differences between the INFP and INFJ personality types. The only true way to determine which one you embody is to study the cognitive functions at length and determine whether you lead with introverted intuition and extroverted feeling (INFJ) or introverted feeling and extroverted intuition (INFP).