From the Latin reflexio (from re-flectere, to bend back), the return of an agent or power upon its own operation or upon itself. Though the concept can apply to any spiritual operation or power (such as the will), it usually refers to knowledge. Four types of cognitive reflection may here be distinguished: (1) psychological, or knowledge one has of one's own operation, power, and self, both in direct singular awareness and in scientific and universal knowledge; (2) epistemological, or consciousness of the grounds for judging, for recognizing the possession of truth, and for certitude; (3) logical, or reflection upon the status of apprehended natures in thought with their consequent properties and relations there; and (4) ontological, or a simple return to the objects of knowledge for further and more detailed inspection and analysis.
Greek and Augustinian Views. Aristotle recognized that there is a certain reflex knowledge even in sense, for man perceives that he sees, hears, etc. (Anim. 425b 12). Self-knowledge more properly belongs to the intellect, which is itself intelligible (430b 24). The highest form of being is pure intelligence, or thought, and its characteristic and exclusive activity is "thinking thought," i.e., knowing itself (Meta. 1074b 33–35; cf. 1072b 19–20, 23).
neoplatonism stressed this self-regarding aspect of intellect, or mind. plotinus said of his second hypostasis, intelligence, that it not only has knowledge, especially of the intelligible world, but that it essentially knows itself and its own act (Enneads 5.3.1–10). It is its own first and proportioned object in which everything else in known. In proclus the doctrine of return upon self becomes more prominent. Everything that returns upon itself must be incorporeal and self-subsistent (Elements of Theology 16–17, 42–43), and such a being returns not only upon its activity but also upon its essence (44), particularly in self-knowledge (83). Every intelligence knows itself immediately (167) and, in knowing anything, by the same act knows that it knows (168). The influence of Proclus reached the Latin West in the late 12th century through the 9th-century Arabian compilation and digest known as the liber de causis, long attributed to Aristotle. There the schoolmen read that an intelligence, in knowing anything else, at the same time knows itself and its own essence (13) and "returns completely to its own essence" (15).
St. augustine, before Proclus but influenced by Plotinus, held for the presence of the soul to itself and a consequent self-intuition (Trin. 9.3.3, 10.7.10, 10.9. 12–13,14.4.7). There is a habitual self-awareness always with man in "memory," or consciousness (14.6. 8–9, 8.11). Man is aware of his activities of remembering, willing, thinking, and judging (10.10.13, 11.14); and even if he doubts or errs, he knows that he is living (ibid. ).
In the Middle Ages the influence of Augustine continued to be strong. St. bonaventure, for example, held that the soul is immediately present to itself and by nature knows itself intuitively (In 2 sent. 19.1.1 sed contra 7; cf. In 1 sent. 3.2.2 ad 1, 2 ad 2; Itin. ment. 3.1–2); for it belongs to the nature of an intellectual substance to know itself in a complete return (De don. Spir. Sanct. 8.20; In Hexaem. 12.16).
Doctrine of Aquinas. The influence of Augustine, of the Liber de causis, and of Aristotle converged in St. thomas aquinas to produce an elaborate doctrine.
Psychological Reflection. Spiritual substances and immaterial powers, because immaterial, have complete reflection (De ver. 22.12; In 2 sent. 19.1.1). This applies to the will as well as to cognition (De ver. 22.12): the will wills its own act and its own good. Even in sense knowledge there is a certain return, as man knows his own sensing, though not by the external senses but by a distinct internal power (In 3 anim. 2.284–87, 13.391; De ver. 1.9, 10.9). Only the intellect has a complete cognitive return, knowing besides its objects its acts, itself, and its own nature (Summa theologiae 1a, 87.3; In 3 sent. 23.1.2;C. gent. 2.49, 4.11). It also knows whatever is in the soul (ST 1a2ae, 112.5 ad 1), especially its means of knowing such as the intelligible species (ST 1a, 85.2) and its habits (De ver. 10.9). Through his intellectual acts too man knows himself and his own existence (De ver. 10.8; In 9 eth. 11.1908; C. gent. 2.75). In this knowledge there is a determinate order: first the direct object is known, then the act of knowing it, and finally the intellect itself (ST 1a, 87.3, 14.2 ad 3).
The soul's concrete awareness of its own act explains man's intellectual knowledge of material singulars. Knowing directly, by abstraction, the immaterial and universal, the intellect can know the singular only indirectly. By knowing its act of apprehending the universal nature, and in this the intelligible species and its abstraction from and dependence upon the phantasm, which presents a singular object, the intellect sees the universal nature in a singular subject. Thus it is by a certain reflection upon the phantasm (reflexio super phantasma ) that the singular is known (De ver. 2.6, 10.5; In 3 anim. 8.712–713; ST 1a, 86.1).
An important distinction is made between the reflection of immediate consciousness and the reflective consideration that yields a quidditative, scientific, and universal knowledge of the soul, the intellect, and its acts (De ver. 10.8, 9; C. gent. 2.75, 3.46; ST 1a, 87.1). In immediate consciousness, what is primarily given is the existence of one's own singular act currently going on; but implicit in this awareness is the existence of oneself, one's soul, and the intellect, and also the nature of the act, of the power, and of the soul. For what is known is an act of a determinate sort, and it is apprehended concretely, as it is; and consequently the subject and principles of this are also implicitly apprehended as of a determinate sort (De ver. 1.9; In 3 sent. 23.1.2 ad 3). But this knowledge is not explicit; it is in simple apprehension only and not in reflexive judgment; and especially it is not universal, i.e., of the nature of understanding, intellect, and soul as such. In explicit, judgmental, and universal reflection the nature, or quiddity, of the soul is known from that of its power, the intellect; that of the intellect, from its act, understanding; and this, from its object. The soul, for instance, is known to be immaterial because the intellect is an immaterial power; the immateriality of the intellect is known from the "quality," or nature of its act; and that of the act from the immateriality of its object, especially because it is a universal nature (see soul, human 4; spirit). Such quidditative psychological knowledge is not spontaneous and easy but "most difficult" (De ver. 10.8 ad 8 contr.) and the result of a "diligent and penetrating inquiry" (ST 1a, 87.1; cf. 2). (see introspection.)
Epistemological Reflection. Spontaneous psychological reflection is the basis of a reflection that gives a knowledge of the possession of truth and consequently yields certitude. In judging, the intellect affirms or denies something of something else. The judgment is known to be warranted because what is apprehended about the thing (the predicate) is known to belong to the thing (the subject); for the intellect knows the act of apprehending and its nature, knows that what it has apprehended is derived from the thing and expresses some aspect of the thing, and consequently knows that it is the nature of its act, and of itself as the principle of the act, to come into conformity with the thing that is made its object (De ver. l.9; In 1 perih. 3.6 and 9; In 6 meta. 4.1236). This is an awareness of truth and the basis of certitude.
Logical Reflection. This differs from psychological reflection in that the "secondary objects of understanding" with which logic is concerned are not the acts, intelligible species, and concepts "according to the existence which they have in the knower," which psychology studies (De ver. 10.4, 2.5 ad 17), but "secondary intentions which follow upon the manner of understanding" (De pot. 7.9; In 4 meta. 4.574; In 1 anal. post. 20.5). They are views of the primary intentions or concepts not considered subjectively but objectively, according to what is represented, that is, the apprehended natures, and the condition that these natures have in the mind as a result of being apprehended, e.g., universality or the fact of being a genus. Formed by the intellect, these intentions are attributed to the natures that are known, not as these natures exist in reality, i.e., in singular things, but precisely as known, i.e., as existing in the mind (De pot. 7.6).
Modern Doctrines. The whole philosophy of R. descartes is built upon self-awareness, or reflection. Its basic principle, which alone is found indubitable, is Cogito ergo sum: "I think, therefore I am" (Discourse 4; Medit. 2; Prin. 1.7). By thinking he means every event of immediate consciousness within man, such as understanding, willing, imagining, and sensing (Prin. 1.9; Medit. 2, 3; Replies 2 Arg. def. 1; 3.2). Thought is known by the mere fact of thinking; and even when all else is doubted, in the act of doubting the fact of thinking cannot be doubted; and at the same time the thinker is known as existing and as a thinking thing (res cogitans ), a substance whose whole essence, or nature, is simply to think (Discourse 4; Medit. 2, 3; Replies 2.4; 5.2.4; Prin. 1.7). Descartes even refers to the self simply as thought (cogitatio ), meaning, however, not simply the operation but the thing or nature so operating (Replies 3.2; For Arnauld, July 29, 1648). There is accordingly much ambiguity in his usage of the word "thought"; and thinking for him is not distinctively intellectual. The self-awareness that attends it, however, is intellectual and spiritual.
J. locke makes more direct mention of reflection as a principle in his philosophy than does Descartes, for he lays down as the exclusive sources of man's knowledge sensation and reflection (Essay 2.1.2–4). Reflection is "the perception of the operations of our own mind within us, as it is employed about the ideas it has got," that is, such operations as "perception, thinking, doubting, believing, reasoning, knowing, willing" (ibid. ) and "remembrance, discerning, reasoning, judging, knowledge, faith, etc." (2.6). Simple ideas of these operations are formed, but these ideas as such seem to play relatively little part in the subsequent explanation of knowledge. And reflection has almost no role in the constitution of the objective content of man's knowledge. On the perception of one's own existence Locke sounds almost exactly like Descartes: "If I doubt of all other things, that very doubt makes me perceive my own existence, and will not suffer me to doubt that…. If I know I doubt, I haveas certain perception of the existence of the thing doubting, as of that thought which I call 'doubt'…. In everyact of sensation, reasoning, or thinking, we are conscious to ourselves of our own being" (4.9.3). This self-awareness does not lead Locke, however, to any ample knowledge of the nature of the soul; for man is "in the dark concerning these matters" (2.27.27).
G. W. von leibniz defines reflection as "an attention to that which is in us" (New Essays, Introd. 4) and holds that it is by reflection, or self-consciousness, that the human soul is distinguished from inferior monads (Monadology 30; cf. 15, 19, 23, 29). The soul or mind, moreover, contains the whole universe within itself and is a mirror of the whole and an image of God (Principles of Nature and of Grace 13–14; Monad. 56–57, 60–62). Thus through reflection upon itself and reasoning it knows everything else. Attractive as this doctrine may be, it takes insufficient account of man's experience of the process of knowing and of the effort and steps involved.
Phenomenology and Existentialism. Reflection of a somewhat different sort plays an important role in phe nomenology, a philosophy or method of philosophizing that professes to be a radical, unprejudiced, and detailed description of experience or "pure consciousness," particularly as propounded by E. husserl. The data of experience are scrutinized precisely as experience; neither the objects nor the subject are considered in their ontological reality but only as exhibited or manifested: objects are viewed only as objects and the subject just as a center of pure consciousness evoked and conditioned by what it is conscious of. This requires a reflection that is called "transcendental" and is distinguished from "natural" reflection, which includes both immediate consciousness of one's acts of perception, recall, and predication as well as the studied introspection of psychology; this new type observes and makes explicit the content of these acts of consciousness (Cartesian Meditations 1.15; Ideas 38, 45, 77–78, 108, 150) from the viewpoint of its formal structure, essence, or "meaning" (Ideas 3–5, 129–131). This would make the reflection ontological rather than psychological, except that it is not concerned with the existence, or reality, of the object. And this reflection reveals the transcendental "I," which is not "a human Ego in the universal, existentially posited world, but exclusively a subject for which this world has being" (Ideas, Introd. to Eng. ed.; cf. 33, 46, 49, 50, 80, 92, 115; Cartes. Med. 1.11; 4.30–33, 36). The subject is constituted in constituting the object, the world of its own experience. For Husserl himself, phenomenology tended toward idealism, though this does not seem to be a necessary consequence. The faithful acceptance of the data of experience and the careful analysis and description of its contents can prove a solid beginning for philosophy; but its sufficiency for a complete philosophy has been questioned or denied by most of his followers, who have gone on in the various directions of psychology, psychoanalysis, ethical inquiry, and especially existentialism (see M. merleau ponty, "What Is Phenomenology?" Cross Currents 6 (1956) 59–70).
In existentialism attention is focused not only on human existence but on the experience of this existence. S. A. kierkegaard stresses the subjectivity of the existence of the individual, which is known by direct experience but cannot be expressed to another in universalized and systematized thought. Each one not only knows his own existence but must be himself (Either/Or 2, 150); but the "immediate" man, immersed in other things and without reflection, cannot do this (Sickness unto Death 80–81). Reflection, which is presupposed by consciousness (Johannes Climacus 2.1, 151), is needed to bring out distinctness and individuality (ibid. 150–53) and also to attain the inwardness of a conscious, ethical, and religious man (Postscript 169–224). Existentialist philosophers have tended to place a chasm between subject and object and, in stressing subjective knowledge, to eliminate or minimize essences, thereby abolishing the principle of intelligibility of things. Gabriel Marcel has resisted this tendency. He considers thought as self-transcendence (Being and Having 30) and propounds a twofold reflection. Primary reflection is turned toward the objective content of knowledge as independent of the act of thinking in order to inspect and analyze; it could accordingly be classified as ontological. Secondary reflection, directed toward the primary, considers the content precisely as given in the act of experiencing and, without denying the value of primary reflection, recuperates the original unity and reveals the self-transcendence of the self and founds objectivity (Du refus à l'invocation 34; Philosophy of Existence 14).
See Also: intentionality; knowledge, process of; species, intentional.
Bibliography: j. p. ruane, "Self-Knowledge and the Spirituality of the Soul in St. Thomas," New Scholasticism 32 (1958) 425–442. j. d. mckian, "The Metaphysics of Introspection According to St. Thomas," ibid. 15 (1941) 89–117. g. p. klubertanz, "St. Thomas and the Knowledge of the Singular," ibid. 26 (1952) 135–166. c. v. pax, "Philosophical Reflection: Gabriel Marcel," ibid. 38 (1964) 159–177. b. romeyer, Saint Thomas et notre connaissance de l'esprit humain (2d ed. Archives de philosophie 6.2; Paris 1932). j. de finance, Cogito carsésien et réflexion thomiste (ibid. 16.2; 1946); "Being and Subjectivity," Cross Currents 6 (1956) 163–178. j. wÉbert, "'Reflexio': Étude sur les opérations réflexives dans la psychologie de Saint Thomas d'Aquin," Mélanges Mandonnet, 2 v. (Bibliothèque Thomiste 13, 14; 1930) v. 1.
[r. w. schmidt]
reflection, return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the reflection of light and sound are similar, for both normally travel in straight lines and both are wave phenomena. Objects are visible because of the light reflected from their surfaces, and their color depends on their ability to reflect light of a certain wavelength and to absorb that of other wavelengths. The reflection of sound waves from a surface is called an echo.
The Laws of Reflection
The reflection of light follows certain definite laws. A ray of light striking a reflecting surface at right angles to it is returned directly along the path it has followed in reaching the surface. When, however, a ray strikes a reflecting surface at any other angle, it is reflected at an angle in an opposite direction. The incoming ray is called the incident ray. Its direction is usually described by the angle of incidence, which is the angle that it makes with the normal, or line perpendicular to the reflecting surface at the point of reflection. The angle formed by the reflected ray and the normal is called the angle of reflection and is equal to the angle of incidence. Furthermore, the reflected ray is always in the same plane as the incident ray, and this plane is perpendicular to the surface.
The Degree and Types of Reflection
Not all surfaces reflect light in the same way or to the same degree. The measure of the fraction of light that is reflected by a material is called its reflectance. Metals in general have high values of reflectance; silver, for example, has a reflectance of about 96%. Smooth surfaces give regular reflection, also called specular reflection, in which incident parallel rays remain parallel after reflection. Rough or uneven surfaces give diffuse reflection, since the reflected rays are scattered and not parallel. For example, reflection by a mirror is regular; by a highly polished but uneven piece of metal, it is diffused. Reflection of light is also brought about under certain conditions by the surfaces of transparent media through which light normally passes. An example is seen in the blazing glare of sunlight on a window or an automobile windshield when the sun's rays strike it at a very oblique angle.
A corner reflector returns a ray that is exactly parallel to the incident ray back to the incident ray's point of origin, or very close to it. The reflector is formed by intersecting three mutually perpendicular planes, with the centerpoint therefore being located at the mutual point of intersection. Such a device can be utilized as a radar target or marker for range finding and surveying. For increased visibility at night microscopic corner reflectors can be incorporated into reflective paint for road signs and incorporated into the lenses of bicycle and motorcycle reflectors. Several U.S. Apollo missions and one Soviet Lunakhod lunar probe deployed corner reflector arrays on the lunar surface. When the arrays are illuminated by laser beams originating from the earth, precise measurements of the roundtrip travel time of the light permit the calculation of the earth-moon distance to an accuracy of 6 in. (15 cm). Such measurements also are used to determine the moon's orbit with greater accuracy, to record perturbations in the moon's motion caused by meteorite impacts, and to ascertain the length of an earth day.
The phenomenon called total internal reflection is observed when light passing from one medium (e.g., a glass prism or water) to a less dense medium (e.g., air) reaches the boundary between the two media and is thrown back into the denser medium instead of passing outward as would be expected. This occurs when the light strikes at an oblique angle, greater than a certain degree. Up to that degree, refraction (not reflection) takes place, and the greatest angle at which refraction is possible is called the critical angle; if the angle of incidence exceeds this angle, total reflection occurs. The fire of a faceted diamond is due to total internal reflection. Internal reflection accounts in part for a number of natural phenomena. Rays of sunlight striking raindrops are refracted on entering them and then undergo internal reflection; since the sunlight is broken up into its colors, a rainbow appears. A mirage is also partially the result of internal reflection.
re·flec·tion / riˈflekshən/ • n. 1. the throwing back by a body or surface of light, heat, or sound without absorbing it: the reflection of light. ∎ an amount of light, heat, or sound that is thrown back in such a way: the reflections from the streetlights gave us just enough light. ∎ an image seen in a mirror or shiny surface: Marianne surveyed her reflection in the mirror. ∎ a thing that is a consequence of or arises from something else: a healthy skin is a reflection of good health in general. ∎ [in sing.] a thing bringing discredit to someone or something: it was a sad reflection on society that because of his affliction he was picked on. ∎ Math. the conceptual operation of inverting a system or event with respect to a plane, each element being transferred perpendicularly through the plane to a point the same distance the other side of it. 2. serious thought or consideration: he doesn't get much time for reflection. ∎ an idea about something, esp. one that is written down or expressed: reflections on human destiny and art.