Who first defined truth as "adæquatio rei et intellectus"?

Question

António Manuel Martins claims (@44:41 of his lecture "Fonseca on Signs") that the origin of what is now called the correspondence theory of truth,

Veritas est adæquatio rei et intellectus.

Truth is the adequation of thing and intellect.

is unknown. Is this true, or was there a philosopher who first stated this definition of truth?

Answer 1

William of Auvergne first introduced the term adæquatio.

Jan Woleński, “Contributions to the History of the Classical Truth-Definition,” in Studies in Logic and the Foundations of Mathematics, ed. Dag Prawitz, Brian Skyrms, and Dag Westerståhl, vol. 134, Logic, Methodology and Philosophy of Science IX (Elsevier, 1995), 481–95. (pp. 487-88):

Let me finish this section [6. Schoolmen] with some historical remarks (see Gilson 1955). Thomas Aquinas notes [in De veritate q. 1 a. 1 co.?] that his definition of truth is derived from Liber de definitionibus by Isaac Israeli; Aquinas also refers to Avicenna in this context. However, adequatio does not occur in Israeli's truth-definition which (in Latin version) is this:

Et sermo quidem dicentis: veritas est quod est, enuntiativus est natura veritatis et essentiæ ejus, quoniam illud sciendum quod est res, vera est; est veritas nonnisi quod est;
_{And the word indeed of the speaker: truth is what is; the enuntiative is the nature of truth and its essence, since it must be understood that what is, is true; truth is only that which is.}

this formula is fairly Aristotelian. Avicenna in his Metaphysics says (in Latin translation) that

veritas […] intelligitur dispositio in re exteriore cum est ei æqualitas;
_{truth […] means a disposition in an external thing when there is equality to it.}

the last word suggests the strong sense of ‘correspondence’. It was William of Auvergne who introduced the term adequatio in philosophy for the first time. He refers (in De universo) to Avicenna in the following way:

[…] et hoc [intentio veritas] ait Avicenna, est adequatio orationis et rerum.
_{and this [intention (is) truth] Avicenna says, is the adequation of speech and things.}

Then William adds that the truth is

adequatio intellectus ad rem.
_{adequation of the intellect to the thing.}

In Albertus Magnus’ treatise De bono we find that truth is adequatio rei cum intellectu [“adequation of thing with the intellect”]. Then comes (10 [De veritate q. 1 a. 2 s. c. 2]).

^{my emphases
cited in: Patricia Moya, “La verdad en Tomás de Aquino,” in Felipe Castañeda, Andrea Lozano, and Nicolás Vaughan, eds., Anselmo de Canterbury – Tratado Sobre La Verdad. Edición Bilingüe (Universidad de los Andes, 2018), p. 247n1.
Woleński enumerates 31 definitions of truth from "2. Aletheia in Old Greek" to "9. The Twentieth Century". Why do 🇵🇱s make such awesome logicians? 😄}

Avicenna precursor of St. Thomas and William of Auvergne

María del Carmen Elvira Torres and Luis Xavier Farjeat, “La noción de ‘verdad’ (ḥaqīqa) en Avicena [980-1037 A.D.],” in Castañeda et al., p. 214:

Hablar sobre la noción de verdad (ḥaqīqa) en Avicena implica detectar cantidad de matices. El término árabe puede significar “verdad” o “realidad”. En estricto sentido se refiere al hecho de que algo exista. De ahí que Avicena suela darle, como se verá más adelante, una connotación ontológica: ḥaqīqa al-shay’ (“lo existente” o, literalmente, “cosa existente”). Lo utiliza también cuando se refiere a las esencias concretas y, por ello, ḥaqīqa puede traducirse, según el contexto, como “lo esencial” o para indicar la naturaleza de algo. Tiene además el sentido de algo que, dado que existe de manera real, es conocido por la mente como algo verdadero. En este último sentido, ḥaqīqa es la adecuación de la mente con la cosa (shay’) tal y como es. El término tiene, de esta manera, un sentido también lógico (ḥaqīqa al-‘aqliyya, “verdad inteligida”, en los términos de Avicena, o “conceptualizada”, como suele traducirse). Al implicar un proceso por el cual la mente identifica la existencia de algo, ḥaqīqa al mismo tiempo tiene una connotación cognitiva.

^{[DeepL transl.:] To speak about the notion of truth (ḥaqīqa) in Avicenna implies detecting a number of nuances. The Arabic term can mean "truth" or "reality." Strictly speaking it refers to the fact that something exists. Hence Avicenna tends to give it, as will be seen below, an ontological connotation: ḥaqīqa al-shay' ("the existent" or, literally, "existing thing"). He also uses it when referring to concrete essences, and thus ḥaqīqa can be translated, depending on the context, as "the essential" or to indicate the nature of something. It has further the sense of something which, since it exists in a real way, is known to the mind as something true. In the latter sense, ḥaqīqa is the adequation of the mind with the thing (shay') as it is. The term thus also has a logical sense (ḥaqīqa al-'aqliyya, "intelligible truth," in Avicenna's terms, or "conceptualized," as it is usually translated). By implying a process by which the mind identifies the existence of something, ḥaqīqa at the same time has a cognitive connotation.}

^{my emphases}

Answer 2

The backward search for the phrase “veritas est adaequatio rei et intellectus” leads to Thomas Aquinas De veritate q. 1 a. 1 co.:

The first reference of being to the intellect, therefore, consists in its agreement with the intellect. This agreement is called “the conformity of thing and intellect” (adaequatio intellectus et rei). In this conformity is fulfilled the formal constituent of the true, and this is what the true adds to being, namely, the conformity or equation of thing and intellect. As we said, the knowledge of a thing is a consequence of this conformity; therefore, it is an effect of truth, even though the fact that the thing is a being is prior to its truth.

Consequently, truth or the true has been defined in three ways. First of all, it is defined according to that which precedes truth and is the basis of truth. This is why Augustine writes: “The true is that which is”; and Avicenna: “The truth of each thing is a property of the act of being which has been established for it.” Still others say: “The true is the undividedness of the act of existence from that which is.” Truth is also defined in another way—according to that in which its intelligible determination is formally completed. Thus, Isaac writes: “Truth is the conformity of thing and intellect” (veritas est adaequatio rei et intellectus); and Anselm: “Truth is a rectitude perceptible only by the mind.” This rectitude, of course, is said to be based on some conformity. The Philosopher says that in defining truth we say that truth is had when one affirms that “to be which is, and that not to be which is not.”

Thomas points to Isaak ben Salomon Israeli (845-940).

Then Leonina (Note to line 186) points to a paper of Muckle.

According to Muckle's paper it seems questionable whether the phrase can be actually found in Isaak’s work. Some scholars ascribe the content of the phrase to Avicenna (see Added below), who is later than Isaak.

The phrase expresses the general opinion of theologians in the 13^th century.

Added: Avicenna's Healing and the Metaphysics of Truth (2018) Footnote 32 quotes Avicenna Ilāhiyyāt I.8.1: As regards truth, one understands by it existence in external things absolutely, and one understands by it permanent existence, and one understands by it the state of the statement or of the belief indicating the state of the external thing, whether it [i.e. the statement or belief] conforms with it [i.e. the external thing], such that we would say, “This is a true [ḥaqq] statement” and “This is a true [ḥaqq] belief.” (My emphases)