Drawing upon Jungian theory, Isabel Myers proposed that for each personality type, the cognitive functions (sensing, intuition, thinking, and feeling) form a hierarchy. This hierarchy represents the person's so-called default pattern of behavior.
The Dominant function is the personality type's preferred role, the one they feel most comfortable with. The secondary Auxiliary function serves to support and expand on the Dominant function. If the Dominant is an information gathering function (sensing or intuition), the Auxiliary is a decision making function (thinking or feeling), and vice versa. The Tertiary function is less developed than the Dominant and Auxiliary, but it matures over time, rounding out the person's abilities. The Inferior function is the personality type's Achilles' heel. This is the function they are least comfortable with. Like the Tertiary, the Inferior function strengthens with maturity.
Jung and Myers considered the attitude of the Auxiliary, Tertiary, and Inferior functions to be the opposite of the Dominant. In this interpretation, if the Dominant function is extraverted, then the other three are introverted, and vice versa. However, many modern practitioners hold that the attitude of the Tertiary function is the same as the Dominant. Using the more modern interpretation, the cognitive functions of the ISTP are as follows:
 Dominant: Introverted thinking (Ti)
Ti seeks precision, such as the exact word to express an idea. It notices the minute distinctions that define the essence of things, then analyzes and classifies them. Ti examines all sides of an issue, looking to solve problems while minimizing effort and risk. It uses models to root out logical inconsistency. ISTPs live in a world of logic, on which they base their decisions. They process information from their auxiliary function to create strategies for action at a moment's notice. They love to examine complicated systems.
 Auxiliary: Extraverted sensing (Se)
Se focuses on the experiences and sensations of the immediate, physical world. With an acute awareness of the present surroundings, it brings relevant facts and details to the forefront and may lead to spontaneous action. This function drives ISTPs to embrace opportunities to plunge headfirst into experiences. It also gives them a keen insight into situations similar to that of the ISTJ. Their curiosity for the world around them gives them a "leap before you look" tendency. They are notorious for taking apart a device to see how it works before considering whether they can put it back together.
 Tertiary: Introverted intuition (Ni)
Attracted to symbolic actions or devices, Ni synthesizes seeming paradoxes to create the previously unimagined. These realizations come with a certainty that demands action to fulfill a new vision of the future, solutions that may include complex systems or universal truths. Never as open in their theorizing as INTPs, ISTPs prefer to keep themselves grounded in the physical situation, using Ni to visualize components and concepts that they cannot see and touch firsthand, such as the wiring in a circuit board.
 Inferior: Extraverted feeling (Fe)
Fe seeks social connections and creates harmonious interactions through polite, considerate, and appropriate behavior. Fe responds to the explicit (and implicit) wants of others, and may even create an internal conflict between the subject’s own needs and the desire to meet the needs of others. A weak point of ISTPs, Fe may lead them to interpret criticisms of their ideas as criticisms of their competence. ISTPs strive to follow the edicts of logic, but when they are under stress, their inferior function can lead them to illogically and stubbornly cling to their own ideas—sometimes to their own detriment, or even when proven wrong.
 Shadow functions
Later personality researchers (notably Linda V. Berens) added four additional functions to the descending hierarchy, the so-called "shadow" functions to which the individual is not naturally inclined but which can emerge when the person is under stress. For ISTP, these shadow functions are (in order):
- Extraverted thinking (Te): Te organizes and schedules ideas and the environment to ensure the efficient, productive pursuit of objectives. Te seeks logical explanations for actions, events, and conclusions, looking for faulty reasoning and lapses in sequence.
- Introverted sensing (Si): Si collects data in the present moment and compares it with past experiences, a process that sometimes evokes the feelings associated with memory, as if the subject were reliving it. Seeking to protect what is familiar, Si draws upon history to form goals and expectations about what will happen in the future.
- Extraverted intuition (Ne): Ne finds and interprets hidden meanings, using “what if” questions to explore alternatives, allowing multiple possibilities to coexist. This imaginative play weaves together insights and experiences from various sources to form a new whole, which can then become a catalyst to action.
- Introverted feeling (Fi): Fi filters information based on interpretations of worth, forming judgments according to criteria that are often intangible. Fi constantly balances an internal set of values such as harmony and authenticity. Attuned to subtle distinctions, Fi innately senses what is true and what is false in a situation.