These days it is common, even on first meeting, to be asked "what's your MBTI?" At the same time, many people still go to have their saju read. These two, which look utterly different at a glance, in fact spring from the same wish—to understand "what kind of person am I?"—and are two different languages for it.
MBTI is a personality-classification tool built on the psychological-type theory of the early-twentieth-century psychologist Carl Jung. It sorts people into sixteen types by combining four axes: the direction of energy (Extraversion E / Introversion I), the way of perceiving (Sensing S / Intuition N), the way of judging (Thinking T / Feeling F), and the way of living (Judging J / Perceiving P). Because it is self-reported—you answer questions to get your result—the same person's result can shift a little with time or circumstance.
Saju is far older: an Eastern tradition that interprets the year, month, day, and hour of birth through the Heavenly Stems and Earthly Branches and yin-yang and the five elements. If MBTI is closer to self-perception—"this is how I feel right now"—saju rests on birth data: "the energy at the moment of birth was like this." So saju differs in grain from MBTI in that it yields a reading from the birth date and time alone, not from your own answers.
What is interesting is that neither is a tool meant to lock a person into a "fixed box." Neither MBTI's sixteen types nor saju's five energies can fully describe a person. Whatever your type, whatever your chart, how you live it out is in the end your own. So confining yourself with "I'm an I, so I can't" or "that's just my saju, nothing to be done" is, for both tools, against their original intent.
The reason many today enjoy MBTI and saju together is that they illuminate you from different angles. If MBTI shows "the way you currently" perceive the world and decide, saju tells of the "larger flow" of your innate temperament and fortune over time. Setting the two languages side by side helps you understand yourself in three dimensions. FortuneLeaf deals with saju and other Eastern fortunes, but its spirit is the same as the attitude toward MBTI: whatever the result, we hope you receive it not as a yardstick to dislike or limit yourself, but as a kind language that helps you understand yourself more generously and choose better.
The decisive difference between the two systems lies in what they rest on. MBTI summarizes into four letters the preferences you yourself reported—the direction of your energy, how you take in information, the basis of your judgment, and your approach to life. That is, "the me that the present me reports." Saju, by contrast, reads the rough sketch of your innate temperament from the moment of birth you did not choose, the Heavenly Stems and Earthly Branches of that instant. One begins from self-awareness, the other from the coordinates of birth. Yet the two share the very same trap: the lure of the label, "I'm just this kind of person anyway." Saying "I can't meet people because I'm an introvert" or "it won't work because of my saju" easily becomes an excuse that halts self-understanding. The healthy use is the opposite. A type is a starting line, not a finish line; knowing your default lets you practice transcending it all the more precisely. Both languages exist not to confine you but to set you freer.