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Form, Metaphysical, in Ancient and Medieval Thought


The metaphysical notion of form (eidos, morphe, Gr.; idea, forma, species, Lat.), as it emerged in the works of Plato, must be carefully distinguished from the everyday notion from which it derived, namely, the shape or outer appearance of a thing as it presents itself to the eyes. The outer appearance of a mannequin, for instance, may be deceptively similar to that of a human being, yet, the form in the philosophical sense would be radically different for the two. The form of the thing in the philosophical sense is what determines the kind of thing it is, and so the kinds of properties it can or cannot have, and the kinds of things it can or cannot do or suffer. The mannequin, being a lifeless artifact, obviously cannot perform the characteristic operations any human being naturally can (unless prevented by some circumstance), such as walking, talking, begetting or giving birth to human offspring, and so forth. Therefore, a human being and a mannequin must have different forms. But does it follow that two individual human beings must have the same form? Two individual human beings are equally humans, and as such they equally have the same sort of characteristic operations and abilities. However, no two human beings have numerically the same operations and abilities. This author's activity of writing this article is not anybody else's activity, although another human being could perform exactly the same sort of activity. Still, the author's activity and the other person's activity are two, hence numerically distinct, activities, even if they are of the same sort. In the same way, the form on account of which the author has the ability to perform this activity is not numerically the same as the form on account of which another person would be able to perform the same sort of activity. So, there must be some numerically distinct individualized forms, namely, forms that individually determine one's essential abilities.

Forms as Universal Exemplars in Plato

Still, these individualized forms, which Plato (c. 428348 or 347 b.c.e.) briefly recognizes in his Phaedo (102d103c), are not what he would call Forms or Ideas in his Theory of Forms. Plato's Forms are rather the universal exemplars after which individualized forms are modeled. Consider, for example, any geometrical shape, say, a sphere. Any spherical thing, such as a pearl, is spherical on account of its own spherical shape (its individualized shape), but all pearls (and all billiard balls, bowling balls, etc.) are spherical because they all have the same sort of shape, as if each were just a copy or imitation of a common, universal model, the Form of Sphere, or Sphericity itself. To be sure, different spherical things may realize their common Form differently, say, with different diameters, and with different sorts of imperfections, but insofar as they all realize the same Form, they all constitute the same sort of objects. Indeed, imitating or participating in the same Form is precisely what Plato would take to be the reason why distinct particulars of the same kind belong to the same kind.

But how does one know about these Forms? In his Phaedo (73c.75c.), Plato presents an interesting argument to show that the ability to recognize things as more or less perfect realizations of their exemplars entails people's souls' prenatal acquaintance with these exemplars. The gist of the argument can be restated as follows. Whenever one sees things that are more or less equal, the ability to recognize them as such and to judge them as being more or less perfectly equal presupposes the acquaintance with absolute, perfect Equality, the Form that all imperfectly equal things are trying to imitate with their imperfect equality. For how else would one know that an equal pair of sticks is not perfectly equal? But perfect equality can certainly never be met in one's sensory experiences. So, this acquaintance with Equality itself cannot be obtained from sensory experience. However, experiences begin with birth. Therefore, people must have their acquaintance with Equality from a prenatal form of existence, from before their souls entered their bodies at birth.

This little piece of reasoning contains all major elements of Plato's philosophical theory in a nutshell. Metaphysically, Forms are the independently existing perfect, universal standards for the perfection of any thing of any given kind. In epistemology, Forms are the source of the possibility of true universal knowledge: by recollecting one's knowledge of their universal exemplar, one has universal knowledge of all particulars that share in the same Form. Finally, in moral and political theory, the realm of Forms is that domain of pure perfection where immortal human souls belong by nature; therefore, one's task in this life is to prepare the soul for its safe return by living one's life according to the standards of perfection set by the Forms.

That the "naïve" Theory of Forms (as presented here) is inconsistent was recognized already by Plato in his Parmenides, where can be found the first formulation of the famous Third Man argument (132ab), proving the inconsistency of the theory.

Consider the Form of Humanity. According to the theory, each human belongs to this species by participating in one and the same Form. But the Form itself is perfectly human, so it also belongs to the species of humans. Therefore, it should also pertain to this species by participating in the Form of Humanity. However, it cannot participate in itself, for what participates is inferior to what it participates in, and nothing can be inferior to itself. So, there has to be another Form of Humanity, which itself would also have to be human, that is, there would have to be a Third Man, besides the particular humans, and the first Form of Humanity. Indeed, the same reasoning could be repeated, yielding an infinite series of Forms for the species of humans. However, the theory also claims that for a species of particulars there is only a single Form in which they all participate, which is inconsistent with the infinity of Forms also implied by the claims of the theory. Therefore, the theory cannot be true as stated.

Individualized Forms in Aristotle

Moved by this and a number of other arguments, Aristotle (384322 b.c.e.) famously rejected Plato's Forms. Hence the adage attributed to Aristotle, usually quoted in Latin: amicus Plato sed magis amica veritas I love Plato, but I love the truth even more. Aristotle did not abandon, however, what has been called above the individualized forms of things. For Aristotle, it is these forms that individually determine each thing's nature, sorting them into their natural kinds, reflected in the Aristotelian system of categories.

Individualized forms are either substantial or accidental. Substantial forms are what account for the individual existence of substances, the independently existing individuals that are not properties of other individuals, such as this man. Accidental forms (or briefly, accidents) are the individualized properties of substances, such as the height or color of this man. Individual substances and accidents are classified in the system of categories into their universal kinds, their species and genera.

But how can individuals have universal knowledge concerning all individuals of the same species and genera if they cannot have the direct acquaintance with their universal exemplars presumed by Plato? Aristotle's answer is his theory of abstraction. In his view, the human intellect is not a merely passive recipient of "ready-made" universal information obtainable from the universal exemplars of particulars, but it is capable of actively extracting this universal information from the experience of several particulars by separating what is common to all and disregarding what is peculiar to each. The human soul, the individualized substantial form of a human body, uniquely has this ability among other living bodies, through the so-called active intellect (nous poietikos, Gr.; intellectus agens, Lat.). This unique ability of the human soul ranks it the highest in perfection among all material forms, linking it to the realm of immaterial forms or separate substances. In fact, some commentators of Aristotle, such as Averroës (Ibn Rushd; 11261198), interpreted Aristotle as claiming that the active intellect itself is such a separate substance. Other, more perfect separate substances are the movers of celestial spheres that by the light of the celestial bodies they hold provide the influx of energy required for the activity of the natural agents on earth (mixing the four elements, and sustaining the generation and corruption of living things). However, Aristotle argues that it is impossible to go in the series of ever more perfect movers moved by even more perfect ones to infinity. So, ultimately all are moved by a first unmoved mover, the Prime Mover, which is therefore the source of the activity of everything else.

Forms as Divine Ideas in St. Augustine

The next important step in the development of the notion form was provided by St. Augustine of Hippo's (354430) Neoplatonic Christian conception. For Augustine, Forms are universal exemplars, just as they are for Plato, but they are not Plato's mind-independent models of various species, for they are the archetypes of creation in the Divine Mind. The Divine Ideas are the models for creatures in the eternal thought of God. Therefore, Augustine argues, a moment of understanding some eternal truth is but a glimpse into divine thought granted to humans by God in an act of illumination. Augustine's conception powerfully combined Christian teaching with elements of Platonic philosophy, but it raised a number of new questions. Especially, in epistemology, it raised the issue of why a natural capacity of the human mind, namely, the understanding of "secular" eternal truths, say, in logic or mathematics, should be regarded as directly dependent on divine grace.

The Syncretic Theory of Forms
of St. Thomas Aquinas

This question, and many others, received a balanced "naturalistic" answer in the Aristotelian Christian synthesis of St. Thomas Aquinas (c. 12241274), reconciling Christian religious doctrine and important elements of the Neoplatonic tradition with Aristotelian philosophy. For Aquinas, divine grace does not work against nature (even if it could), but through nature: gratia non tollit naturam, sed perficit grace does not destroy nature, but perfects it (2SN, d. 9, q. 1, obj. 8; ST1, q. 1, a. 8, resp. 2.). Therefore, the divine light of Augustinian illumination for Aquinas becomes the same as the Aristotelian active intellect, which (pace Averroës) is not a separate substance, but a natural, although immaterial, power of the human soul. The soul itself is the Aristotelian substantial form of the human body, which, however, on account of its immaterial power, is naturally capable of surviving the death of a human person, to be supernaturally resurrected in the same body by God. The human soul thus straddles the ontological divide between material and immaterial, having access to both. The natural world of material substances, each having its natural powers on account of its characteristic substantial form, but all in need of a constant influx of energy for its natural operation, is kept in motion by the movement of heavenly spheres, in accordance with the laws of Aristotelian physics. These in turn are moved by their immaterial movers, Aristotle's subsistent forms or separate substances, which Aquinas further identifies with biblical angels. But even these immaterial forms are not pure actuality or energy (actualitas, Lat.; energeia, Gr.). Therefore, they owe their activity as well as their actual being to the Aristotelian Prime Mover, the first cause of all causes, which for Aquinas is also the creator of the universe, both material and immaterial, namely, the Judeo-Christian God, the subsistent form that is pure energy or actuality, that is, nothing but pure being: He Who Is (ST1, q. 13, a. 11; Exodus 3:14).

See also Aristotelianism ; Platonism .



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. Summa Theologica. A complete English translation, translated by the Fathers of the English Dominican Province. Westminster, Md.: Christian Classics, 1981.

A Summary of Philosophy. Translated and edited, with introduction and glossary, by Richard J. Regan, S.J. Indianapolis: Hackett, 2003. A modern translation of selected philosophical texts from Aquinas's theological masterpiece.

Aristotle. The Complete Works of Aristotle: The Revised Oxford Translation. 2 vols. Edited by Jonathan Barnes. Princeton, N.J.: Princeton University Press, 1984.

Augustine. The Essential Augustine. Edited by Vernon J. Bourke. Indianapolis,: Hackett, 1974. See especially "Divine Ideas as Prototypes," pp. 6162; selection from Augustine's Eighty-three Different Questions, q. 46, 12.

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. Plato's Parmenides. Translated with introduction and commentary by Samuel Scolnicov. Berkeley: University of California Press, 2003.


Davies, Brian. "Thomas Aquinas." In Blackwell's Companion to Mediaeval Philosophy, edited by Jorge Gracia and Timothy Noone, 643659. Oxford: Blackwell, 2002.

Fine, Gail. On Ideas: Aristotle's Criticism of Plato's Theory of Forms. Oxford: Clarendon, 1993.

Klima, Gyula. "Natures: The Problem of Universals." In The Cambridge Companion to Medieval Philosophy, edited by Arthur Stephen McGrade, 196207. Cambridge, U.K.: Cambridge University Press, 2003.

Vlastos, Gregory, ed. Plato: A Collection of Critical Essays. Notre Dame, Ind.: University of Notre Dame Press, 1978.

Gyula Klima

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