Swedenborg's Definition of Rationality
In the Heavenly Doctrines, Swedenborg gives his clearest formal definition in Divine Love and Wisdom:
"A person has in him two faculties from the Lord which distinguish him from animals. One of these faculties is his ability to understand what is true and what is good. This faculty is called rationality, and it is a faculty of his intellect. The other faculty is his ability to do what is true and good. This faculty is called freedom, and it is a faculty of his will." (Divine Love and Wisdom 240)
So at its core, rationality is defined as the human faculty — belonging to the intellect — of being able to understand truth and goodness. It is paired with its complementary faculty, freedom (belonging to the will), which is the ability to actually do what is true and good. Together these two faculties — rationality and freedom — are what make a person human rather than animal, and they remain with every person, good or evil, as the very dwelling-place of the Lord within them, "for if they were taken away, his humanity would perish" (Divine Love and Wisdom 240).
A closely related definition appears in Divine Love and Wisdom 247, where rationality is described as "the faculty of receiving spiritual light," given to every person from the Lord and never removed, since without it a person could not be reformed. It is this faculty that lets a person not only think, but speak in accordance with thought — something animals cannot do.
Key qualifications and elaborations
Rationality as a faculty vs. actually being rational. Everyone possesses the faculty of rationality from birth, but this does not mean everyone actually is rational in practice. Swedenborg distinguishes the inborn capacity from its use: a person becomes genuinely rational only by loving and living according to truth, not merely by knowing it (Heaven and Hell 468).
Three levels of genuine rationality. According to Heaven and Hell 468, true rational ability is built from three kinds of truths — civic (fairness and equity), moral (honesty and uprightness), and spiritual (goodness and truth regarding heaven and the church). A person's rational faculty is "opened" at three corresponding levels, but only through living by these truths from a spiritual affection — loving what is good and true because it is good and true, rather than for self-interest. If truths are loved merely for one's own reputation or profit, they remain external and do not truly open or form the rational mind.
Rational vs. sensuous/merely natural. In Arcana Coelestia 5128, a person is called "rational" when their sensory-level thinking is kept in subjection to deeper truth, and "sensuous" when it is not — someone can appear outwardly rational in speech and action while interiorly being governed by sensory appearances and self-love.
Rational vs. spiritual. Apocalypse Explained 569 clarifies that being rational and being spiritual are not identical: "every spiritual man is also rational, but the rational man is not always spiritual, since the rational is in the natural man... while the spiritual is above the rational, and through the rational passes into the natural."
Why it matters. This faculty of rationality is what allows a person to be reformed and regenerated — to see and acknowledge spiritual truths and freely choose to live by them (Divine Love and Wisdom 247; Divine Providence 168). Someone with genuine inner enlightenment from the Lord "senses instantly" the truth of spiritual realities, such as that God is one, that love is the life of faith, and that doing follows willing — this inner perceptiveness is itself an effect of the rational faculty being opened by heaven's light (Divine Providence 168).
In short, for Swedenborg rationality is not simply cleverness or logical skill, but the God-given capacity of the human mind to see and acknowledge truth — a capacity that becomes genuinely "rational" in the fullest sense only when a person freely chooses to live according to the truths they understand.
