PRINCIPIO. — From primum-capere (caput), it is that which comes first, the origin, the beginning (initium, exordium, origo); it corresponds to the Greek ἀρχή: commencement, beginning. From the immediate etymological meaning there derived the figurative one: p. as that which is fundamental, essential, principal (principe). With this meaning (the temporal reference having become, outside ordinary usage, secondary), the term has enjoyed extensive use in the history of thought.
For the Ionian philosophers, the essential problem is the determination of the ἀρχὴ τῆς φύσεως, ἀρχὴ πάντων (Aristotle, Met., I, 3 ff.), that is, of the «primordial» reality (temporal sense) which stands as the «foundation» (ideal meaning) of corporeal beings. In Aristotle the term assumes important and more precise logical-metaphysical applications. «It belongs to every p. to be that first (τὸ πρῶτον) by which a thing is or comes to be or is known... Nature, the elements, thought, deliberation, substance, and the end are therefore p.» (op. cit., V, 1; and s. Thomas in hunc locum; cf. Aristotle, Phys., I, 5, 188 a 27: «oportet enim principia nec ex se invicem esse, nec ex aliis, et ex ipsis esse omnia»). Consequently, all causes are p.: πάντα γὰρ τὰ αἴτου ἀρχαί: Met., loc. cit.). Aristotle develops in particular the doctrine of logical p.: «the p. of demonstration is the immediate proposition, that is, the one which does not derive from another proposition, ὃς μὴ ἄστιν ἄλλη προτέρα (Anal. post., I, 2, 72 a 7). The immediate p. of demonstration (assimi, τὰ πρότερα, ἄμεσα)» «stand by themselves and must be indemonstrable» (ibid., chaps. 3 and 2). The p. «most certain of all», which cannot be demonstrated, contradiction (v.): «it is impossible for the same thing both to belong and not to belong to the same thing at the same time». Every demonstration ultimately refers back to this p.: «for it is, by its nature (φύσει), the p. of all the other axioms» (Met., IV, 3, 1005 b 19, 32; cf. ibid., 4, and Sum. Theol., 1°-2°, q. 94, a. 2: «Super hoc principium omnia alia fundantur»). In the ontological order, the p. or constitutive moments of corporeal reality are matter and form (Aristotle, Phys., I, 7 ff., V. ILB-
MORPHISM); and the supreme p. of becoming, «on which heaven and nature depend», is the «pure», «per se» act, God (Met., XII, chaps. 6-7, 9).
Scholasticism takes up the term with the wealth of its Aristotelian references (definition in Sum. Theol., 1°, q. 33, a. 1 c.). In addition, in relation to the Trinitarian mystery, the distinction—uncertain in Aristotle—between cause and p. is clarified: whereas a cause always implies in the effect a relation of real dependence in being («causa est per se influens esse in aliud»), p. may also indicate a simple relation of origin-consequence (cf. s. Thomas, De potentia, q. 10, a. 1 ad 10; Sum. Theol., 1°, q. 27).
In modern philosophy, while certain traditional references were abandoned together with the doctrines that inspired them, the use of the term became still broader. The following principal meanings may be distinguished:
a) p. in the logical sense. deduction (v.) demonstration (v.). Within a determinate order or system it is unproved and indemonstrable: first, immediate p. At times, however, it is taken in a broader sense, as a premise that is not necessarily immediate. In addition to the distinction indicated (immediate-mediate p.), the p. is universal if it refers to the transcendental order of reality and therefore holds in every field of knowledge (e.g., the p. of identity, contradiction, and excluded middle); particular or proper if it is limited to a determinate field of reality (e.g., the p. of mathematics). It is rational if it is knowable not only through experience but also rationally (a priori), by intuition and analysis of pure concepts or values of thought; experience (v.). In opposition to positivistic approaches, which hold that every p. is founded exclusively upon and measured by experience, or is due to a free choice for practical purposes or interests, it must be maintained that rational p. are the fruit of intellectual intuition: always related to experience with regard to the real content of their terms (subject and predicate), they depend, in their formal meaning (the relation of belonging between terms, affirmation-negation), upon the original intellectual perception of being and its values. Finally, the p. is purely explicative or analytic if the predicate adds nothing to the subject; synthetic if the predicate (whether a priori or a posteriori) adds a determination not formally contained in the subject (cf. E. Kant, Kritik der reinen Vernunft. Transzend. Logik, l. II, chap. 2).
b) p. in the gnoseological sense: it indicates that which, knowledge (v.), is held to be self-evident and true, and therefore the foundation of a critical system of knowing. According to the various approaches, this p. is sometimes located either in an immediate datum of experience (self-consciousness, sensation, moral experience, etc.) or in an abstract logical-metaphysical p.
c) p. in the real-ontological sense. It is an entity or reality upon which another reality depends. cause (v.) is a p. in this sense: extrinsic p., the efficient and final cause; intrinsic p., that is, constitutive of reality itself, the material and formal cause. In Christian thought God is the efficient-transcendent p., and therefore not constitutive, of all reality. monism (v.) Absolute (v.) is the immanent p., and therefore also constitutive, of the finite. Intrinsic-ontological p., rather than «realities», should be understood as metaphysical «moments» of reality (principia «ut quibus»). In Thomism, the sole utterly simple reality, without any composition of constitutive p., is God; every finite being, on the other hand, entails a composition of metaphysical p. (act-potency, essence-being, matter-form, substance-accidents).
d) P. as «maxim». It indicates the general directive rules of action, expressed in judgments of value: aesthetic, ethical, juridical, political, religious p., etc. «Intellectus principiorum» (in the ethical field «synderesis», [v.]) is what s. Thomas calls the natural aptitude and disposition of the mind to formulate the first speculative-practical p. (Sum. Theol., 1°, q. 79, a. 12; cf. 1°-2°, q. 51, a. 1, and In Ethic. ad Nicom., l. VI, lect. 5°, n. 1178-79).
e) p. in the psychological sense. It is the source or cause of psychic activities: person (v.) as «principium quod», or subject to which the action is attributed; nature, the soul, faculty (v.) as «principium quo», or the ontological root and determining condition of the action.
f) P. as a general explanatory law or hypothesis (or directive-methodological rules for the investigation) of the facts and phenomena of nature, e.g.: the p. of the conservation of energy, the p. of inertia, the p. of indeterminacy, etc.
g) In the general sense, p. is that which grounds, explains, or accounts for a reality or a fact, e.g.: vital p., p. of individuation (v.). When said of a science, a system, or a doctrine, p. means its fundamental element or truth (cf. Descartes, Principia philosophiae; Berkeley, Principles of human knowledge, etc.).
In the technical sense, in Fichtean transcendental idealism the absolutely first p. expresses «that act which does not appear, nor can appear, among the empirical determinations of our consciousness, but rather lies at the basis of every consciousness and alone makes it possible». Fichte locates this p. in the Ego as absolute self-positing (Wissenschaftslehre, part 1°, § 1; Italian trans. by A. Tilgher, 2nd ed., Bari 1925, p. 51 ff.).