The MBTI personality inventory is based on the idea that behavior is generally internally consistent based on the ways that the person perceives and interprets the world. Grounded in Jungian theory, the test uses four different dimensions to define 16 personality archetypes. Each personality type is considered equal, but each has its individual perspective on the world. Within this framework, each person has one dominant personality dimension and one that is shown to the world, which may or may not be the dominant dimension. Each dimension is further shaded by its interactions with other dimensions.

The INFJ personality type is introverted rather than extroverted. This individual relies on intuition to interpret surroundings rather than focusing on hard data the senses provide. When making decisions, the person relies largely on the feelings and concerns of the people involved rather than thinking through the situation on a purely logical basis. When interacting with the outside world, this person prefers structure and order, indicative of the "judging" characteristic, rather than feeling through each situation, or "perceiving."

The ISFJ is identical to the INFJ on every dimension but one. Like the INFJ, this partner is introverted, concerned about the feelings of others, and most comfortable with order and structure. The difference is that while the INFJ applies intuition to incoming sensory data, the ISFJ prefers to focus on the physical reality of the here and now. ISFJs prefer facts over interpretations and are drawn to the practical implications of new data.

According to PersonalityDesk, a large volume of research shows that compatibility on as few as two dimensions can correspond to long-term happiness. The data shows that 67 percent of SFJ-NFJ relationships, regardless of introversion or extroversion, are happy. If you are in an INFJ-ISFJ relationship, you have several inherent strengths. Both introverts, you are not threatened by each other’s need for alone time. You are both focused on the needs of everyone in any decision-making process, and you both feel most comfortable in structured environments.

The single difference between an INFJ and an ISFJ can be the source of a great deal of conflict and strife. While the INFJ applies intuition and interpretation to incoming data, the ISFJ prefers to put off analysis in favor of taking in the facts as they occur. You might feel as if you are speaking different languages as your interpretation and recall of each situation are often wildly different. It is critical for both of you to slow down, communicate clearly and understand your partner’s perspective.