Reason is one of the two faces of the mind, the other being intelligence.
It is doubly subjected:
— to the object it observes,
— to the logic that governs its operations.
Reason stands to the conceivable as intelligence stands to the intelligible, a far vaster—if not unlimited—domain.
The conceivable is an abstraction and a construction (concepts are generated and developed), whereas the intelligible is reception, a mirror (speculum) of meaning.
Reason operates according to the hypothetico-deductive mode: it develops knowledge and maintains a distance between subject and object;
intelligence, by contrast, is the faculty of noesis, knowledge by participation, identification of the known, the knower, and the act of knowing.
More precisely
Reason (ratio) is the discursive instrument of the mind.
It proceeds step by step, through successive operations (analysis, comparison, inference) within a space governed by logical principles—chief among them, the principle of non-contradiction.
It excels at establishing relations, deducing, demonstrating; it builds coherent conceptual systems; it formulates procedures and rules.
Its function is mediatory: it orders, clarifies, distinguishes.
But this power is also its limit:
reason can grasp only what is conceptualizable.
It remains confined within what is representable and, in doing so, remains external to its object.
The reasoning subject is never one with what it knows.
Conversely, intelligence (intellectus) operates by intuition:
it receives, without discursive mediation, the essence of what it knows.
It does not construct; it receives.
Its knowledge is participatory: it implies a principial unity of the knower, the known, and the act of knowing.
Discursive logos proceeds; intelligence sees.
We may say that reason opens access to concepts, whereas intelligence opens access to meaning.
Reason uncovers structures; intelligence reveals the source.
Thus, reason is necessary—and precious—for the ordering of knowledge, yet remains subordinate to intelligence, which alone reaches the intelligible as act.
See the paper Reason and Intelligence, the Two Sides of the Mind.
Further reading
Plato, Republic; Phaedrus — On dialectic and the hierarchy of modes of knowledge.
Aristotle, De Anima — On the agent and possible intellect.
Thomas Aquinas, Summa Theologiae — On the distinction between discursive reason (ratio) and intellect (intellectus).
Nicholas of Cusa, De docta ignorantia — On the limits of reason and its intellectual surpassing.
Descartes, Rules for the Direction of the Mind — On the methodological role of reason.
Leibniz, New Essays — On the distinction between truths of reason and contingent truths.
Kant, Critique of Pure Reason — On the constitutive limits of discursive reason.
Husserl, Ideas — On eidetic intuition.
Jean Borella, The Crisis of Religious Symbolism (Paris, L’Harmattan, 2008) — On the essential distinction between reason and intelligence, and on intellect as the sense of being.
Bruno Bérard, Metaphysics for Everyone (Angelico Press);
— Métaphysique pour tous (Paris, L’Harmattan, 2022)
— Sui sentieri della metafisica (IT)
— ¿Qué es la metafísica? (ES)
— Was ist Metaphysik? (DE)
— On the distinction reason / intelligence and the hierarchy of faculties.