A trichotomy is a three-way classificatory division. Some philosophers pursued trichotomies.
Important trichotomies discussed by Aquinas include the causal principles (agent, patient, act), the potencies for the intellect (imagination, cogitative power, and memory and reminiscence), and the acts of the intellect (concept, judgment, reasoning), with all of those rooted in Aristotle; also the transcendentals of being (unity, truth, goodness) the requisites of the beautiful (wholeness, harmony, radiance).
Kant expounded a table of judgments involving four three-way alternatives, in regard to (1) Quantity, (2) Quality, (3) Relation, (4) Modality, and, based thereupon, a table of four categories, named by the terms just listed, and each with three subcategories. Kant also adapted the Thomistic acts of intellect in his trichotomy of higher cognition — (a) understanding, (b) judgment, (c) reason — which he correlated with his adaptation in the soul's capacities — (a) cognitive faculties, (b) feeling of pleasure or displeasure, and (c) faculty of desire[1] — of Tetens's trichotomy of feeling, understanding, will.[2]
Hegel held that a thing's or idea's internal contradiction leads in a dialectical process to a new synthesis that makes better sense of the contradiction. The process is sometimes described as thesis, antithesis, synthesis. It is instanced across a pattern of trichotomies (e.g. being-nothingness-becoming, immediate-mediate-concrete, abstract-negative-concrete); such trichotomies are not just three-way classificatory divisions; they involve trios of elements functionally interrelated in a process. They are often called triads (but 'triad' does not have that as a fixed sense in philosophy generally).
Charles Sanders Peirce built his philosophy on trichotomies and triadic relations and processes, and framed the "Reduction Thesis" that every predicate is essentially either monadic (quality), dyadic (relation of reaction or resistance), or triadic (representational relation), and never genuinely and irreducibly tetradic or larger.
In theology, and in philosophy related to it, trichotomy is the belief that man consists of three parts; a body, soul, and spirit. This stands in stark contrast to dichotomy.
| Plato's Tripartite soul | Rational. Libidinous. Spirited (various animal qualities). |
| St. Augustine's 3 Laws[3] | Divine Law. Natural Law. Temporal, positive, or human Law. |
| St. Augustine's 3 characterizations of the soul[4] | Memory. Understanding. Will. |
| Aquinas's 3 causal principles[5] (based in Aristotle) | Agent. Patient. Act. |
| Aquinas's 3 potencies for intellect[5] (based in Aristotle) | Imagination. Cogitative power (or, in animals, instinct). Memory (and, in humans, reminiscence). |
| Aquinas's 3 acts of intellect[5] (based in Aristotle) | Conception. Judgment. Reasoning. |
| Aquinas's 3 transcendentals of being[5] | Unity. Truth. Goodness. |
| Aquinas's 3 requisites for the beautiful[5] | Wholeness or perfection. Harmony or due proportion. Radiance. |
| Albertus Magnus's 3 Universals[6] | Ante rem (Idea in God's mind). In re (potential or actual in things). Post rem (mentally abstracted). |
| Sir Francis Bacon's 3 Tables[7] | Presence. Absence. Degree. |
| Thomas Hobbes's 3 Fields | Physics. Moral Philosophy. Civil Philosophy. |
| Johannes Nikolaus Tetens's 3 powers of mind[2] | Feeling. Understanding. Will. |
| John Dryden's 3 ways of transferring | Metaphrase. Paraphrase. Imitation. |
| Kant's 3 faculties of soul[1] | Faculties of knowledge. Feeling of pleasure or displeasure. Faculty of desire (which Kant regarded also as the will). |
| Kant's 3 higher faculties of cognition[1] | Understanding. Judgment. Reason. |
| Kant's 3 judgments of quantity | Universal. Particular. Singular |
| Kant's 3 categories of quantity | Unity. Plurality. Totality |
| Kant's 3 judgments of quality | Affirmative. Negative. Infinite |
| Kant's 3 categories of quality | Reality. Negation. Limitation. |
| Kant's 3 judgments of relation | Categorical. Hypothetical. Disjunctive. |
| Kant's 3 categories of relation | Inherence and subsistence. Causality and dependence. Community. In other words: Substance and accident. Cause and effect. Reciprocity. |
| Kant's 3 judgments of modality | Problematical. Assertoric. Apodictic |
| Kant's 3 categories of modality | Possibility. Existence. Necessity |
| Hegel's 3 dialectical moments | Thesis. Antithesis. Synthesis. |
| Hegel's 3 Spirits[8] | Subjective Spirit. Objective Spirit. Absolute Spirit. |
| Charles Sanders Peirce's 3 categories | Quality of feeling. Reaction, resistance. Representation, mediation. |
| C. S. Peirce's 3 universes of experience | Ideas. Brute fact. Habit (habit-taking). |
| C. S. Peirce's 3 orders of philosophy | Phenomenology. Normative sciences. Metaphysics. |
| C. S. Peirce's 3 normatives | The good (esthetic). The right (ethical). The true (logical). |
| C. S. Peirce's 3 semiotic elements | Sign (representamen). Object. Interpretant. |
| C. S. Peirce's 3 grades of conceptual clearness | By familiarity. Of definition's parts. Of conceivable practical implications. |
| C. S. Peirce's 3 active principles in the cosmos | Spontaneity, absolute chance. Mechanical necessity. Creative love. |
| Gottlob Frege's 3 realms of sense[9] | The external, public, physical. The internal, private, mental. The Platonic, ideal but objective (to which sentences refer). |
| Karl Popper's 3 worlds[10] | Physical things and processes. Subjective human experience. Culture and objective knowledge |
| James Joyce's 3 aesthetic stages[11] | Arrest (by wholeness). Fascination (by harmony). Enchantment (by radiance). |
| Louis Zukofsky's 3 aesthetic elements[12] | Shape. Rhythm. Style. |
| Søren Kierkegaard's 3 Stages[13] | Aesthetic. Ethical. Religious. |
| Edmund Husserl's 3 Reductions | Phenomenological. Eidetic. Religious. |
| Maurice Merleau-Ponty's 3 fields[14] | Physical. Vital. Human. |
| Maurice Merleau-Ponty's 3 categories[14] | Quantity. Order. Meaning. |
| Alan Watts's 3 world views | Life as machine (Western). Life as organism (Chinese). Life as drama (Indian). |