[Journal für Philosophie & Psychiatrie, Juli 2011, Supplement]
1. The 19th century psychological context
1.1. The issue of unity of conscious experience in the late 19th century German psychology
1.1.1. The psychological foundation the experience of the relationships of between sensory contents
At the end of the 19th Century, the question of the experience of the relationships between psychical contents becomes a critical issue in German psychology. A same fundamental idea is developed by many authors: perceiving a grouping of single contents as a coherent unit implies the ability to compare the contents, and not only to perceive each one of them individually (Ash, 1995; Romand and Tchougounnikov, in press). In other words, the diversity of conscious phenomena cannot be appreciated spontaneously: the capacity of experiencing relationships between psychical contents is always mediated by an act of thought. Thus, there should be a category of psychical factors whose property is to ensure the unity of conscious experience. In addition to the representations (Vorstellungen) that express a content of a sensory nature, there are presumptive mental entities which express a content of a relational nature. The issue of the relational psychical phenomena was systematically investigated by Stern in his abovementioned dissertation on Psychology of change apprehension (Stern, 1898; summarised in Stern, 1897b). In this monograph, Stern tries, on the basis on both theoretical and empirical arguments, to identify the elementary psychical phenomena which are in charge of our ability to experience the various forms of change (i.e., succession, continuity, discontinuity, progressivity, permanency, etc.) and which also contribute to the structuring of our psychical life in order to make it a whole set of interrelated psychical events. According to him, three categories of psychical factors allow us to experience changes: the transitional signs, the acts of comparison, and the psychical time of presence. By "transitional signs" (Übergangszeichen) Stern refers to a kind of sensation which is supposed to occur during the passage from a mental content to another in order to make us experience it in a particular way. Transitional signs allow us to apprehend indirectly the relationships between consecutive conscious events: they are symbols from which we are able to interpret change. Motion and novelty are two examples of changes that Stern assumes to be perceived by means of specific transitional signs. Stern's transitional signs are closely related to Exner's "sensations of motion" and to various categories of "feelings" which were discussed by Lipps ("feeling of tension", "feeling of satisfaction", etc.) (Exner, 1894; Lipps, 1904). The so-called "acts of comparison" (Vergleichungsakte) would be in charge of the apprehension of the relationships between perceptual episodes we experience separately at different moments of conscious life. As mentioned above, the "psychical time of presence" hypothesized by Stern is a temporally-extended act of consciousness that allows us to perceive mental phenomena in the form of a unified and homogeneous content. This is the psychical activity by which we apprehend the succession of mental phenomena in consciousness and that defines the elementary structure of conscious experience. The time of presence is the most fundamental of the three categories of factors distinguished by Stern, since it constitutes a prerequisite to all other kinds of change apprehension. For Stern, the question of time perception is actually inseparable from the problem of the unity of conscious experience. Before analyzing the theory of the psychical time of presence, it appears also necessary to replace it in the context of the late 19th century's theoretical debate on the psychological foundations of the unity of consciousness. The nature of this debate and its evolution will be studied here from the contribution of three authors: Fechner, Stumpf, and Ehrenfels.
1.1.2. Fechner's "sensations of difference"
Fechner's theory of the « sensations of difference » (Unterschiedsempfindungen) can be considered as a pioneering contribution to the issue of apprehension of relationships between mental contents. In the second volume of the Elemente der Psychophysik (1860), Fechner (1801-1887) discusses the problem of the perception of qualitative differences between the sensations (Fechner, 1860). The theory he defends is mainly based on an analysis of simultaneous contrast. Simultaneous contrast phenomenon was extensively explored during the 19th century. Like many other psychologists of his time, Fechner tried to understand why some colors produce a more vivid impression as soon as they are perceived as being spatially close to each other. According to the interpretation proposed in the Elemente, the effect of contrast between two different hues is by no way the result of an alteration of the intensity of color sensations. It does not directly derive from the sensations themselves, but from the apprehension (Auffassung) of their qualitative difference. In Fechner's opinion, perception of contrast depends on the manifestation of an autonomous psychical act:
"The apprehension of a difference of sensations is a particular act of consciousness which (...) is not given with the sensations themselves but requires particular conditions to occur. We can assert that it is an act of consciousness that is superior to the simple apprehension of a sensation, insofar as it implies a comparison between various sensations, i.e., the consciousness of their mutual relationships." 
The psychical act through which sensory data are compared is what Fechner calls a "sensation of difference". These sensations are "abstract" acts of consciousness, i.e., mental phenomena devoid per se of any sensory content: they correspond to a "second-rate consciousness" (Bewusstsein der zweiten Ordnung) that is added to a more elementary form of consciousness. As sensory data can be compared by a second-rate consciousness, sensations of difference may be compared in the same way by a "third-rate consciousness", and so on. Visual contrast is not the only perceptual phenomenon for which Fechner claims the existence of sensations of difference. According to him, perception of harmony is determined by psychical factors which make possible the comparison between single tones. Sensations of difference, as Fechner emphasizes, play a critical role in the completion of our experience: without them, we would only perceive the sensible world as a chaos of undifferentiated sensory data.
1.1.3. Stumpf's "sensory judgments"
Fechner's thoughts on the qualities of difference will be developed by Carl Stumpf (1848-1936) in his so-called "theory of the relativity of sensations" (Lehre der Relativität der Empfindungen). In his Tonpsychologie (Stumpf, 1883, pp. 3-22), Stumpf insists on the fact that sensations are not experienced singly, but always in relation with other sensations:
"Sensations may exist in the soul without being differentiated from each other. However, it is only by differentiating them from each other and by putting them in relation to each other that they reach our consciousness." 
According to Stumpf, perceptual experience is determined by the apprehension (Auffassung) of relations, differences, or changes between sensory elements: it is the result of sensory judgments (Sinnesurtheile). Stumpf considers judgment as a psychical act necessary for the attainment of a unified perception of reality.
1.1.4. Ehrenfels' "structural qualities"
As from the 1890s, a number of psychologists argue that there is a certain category of mental entities specifically involved in our perceiving of the relationships between sensory data (Ash, 1995, Romand and Tchougounnikov, in press). Such an idea is notably advocated by Christian von Ehrenfels (1859-1932) in his famous paper "Über Gestaltqualitäten" (Ehrenfels, 1890). Despite a style and an argument sometimes unclear, Ehrenfels's text constitutes a significant theoretical contribution, and it is recognized to have exerted a strong influence on the evolution of the psychological thought (Ash, 1995). In this paper, Ehrenfels claims that a perceptual content is not simply a gathering of sensory qualities, but a structure (Gestalt) determined by particular psychical factors, the so-called structural qualities (Gestaltqualitäten) . More precisely, he defines the structural qualities as:
"(…) these positive representational contents which are bound with the representation complexes that occur in consciousness, and which themselves consist in elements that are separable from each other (i.e., that cannot be represented independently from each other)." 
In other words, one should assume the existence of psychical contents whose raison d'être is to bind together other psychical contents, and thus to determine the appearance of a unitary conscious experience. Ehrenfels acknowledges that the structural qualities hypothesis is much indebted to Mach's ideas. In the Analyse der Empfindungen (1886), Ernst Mach (1838-1916) asserts that the spatial structures (Raumgestalten) and the tonal structures (Tongestalten), i.e., the melodies, are immediately perceivable data: they are experienced spontaneously, independently from any reasoning, in the same way as the elementary sensory impressions (Mach, 1886/1959). Although they are complex grouping of sensations, the spatial and the tonal structures are always apprehended as individualized entities. From the psychical point of view, they must have something new, something irreducible, vis-à-vis the sensory elements that compose them. For Mach, the "holistic" character of the spatial and the temporal structures is clearly evidenced by the fact that they remain relatively unaltered when their components are changed. Thus, for instance, a melody is still perfectly recognizable when it is modulated. Similarly, as Mach observes, despite differences in size or color, two similar geometrical figures can be spontaneously identified as being a unique form. The fact that melodies and figures remain identifiable despite the modification of their sensory components proves, according to Ehrenfels, that there are some psychical elements capable of structuring in a particular way our perceptual contents. A perceptual content is not simply a sum of sensory components, but the result of the association between a complex of representations (Vorstellungscomplex) and a structural quality. Though corresponding to a category of mental entities distinct from sensory qualities, structural qualities are never experienced separately from the complexes of representations: they are spontaneously given together with a particular "substratum" (Grundlage). Ehrenfels distinguishes various kinds of structural qualities, each of them relating to a kind of substratum of its own. According to him, structural qualities are divided into two main categories: the non-temporal structural qualities (unzeitliche Gestaltqualitäten) and the temporal structural qualities (zeitliche Gestaltqualitäten). The first category consists in structural qualities whose substratum is composed of sensory elements that are immediately given in perceptual experience. Non-temporal structural qualities determine the apprehension of spatial relationships and then relate specifically to visual and tactile sensations. They are also involved in experiencing non-extensive relationships between tones (harmony and timbre) and colours (harmony and disharmony of colours). The second category consists of structural qualities whose substratum is composed of both perceptual representations (Wahrnehmungsvorstellungen) and memory (Erinnerungs-) or expectancy images (Erwartungsbilder), i.e., of sensory contents that are immediately perceived, and of sensory contents that have already been or will be perceived. Temporal structural qualities are immediate acts of consciousness that permit the simultaneous seizing (gleichzeitiges Umfassen) of the different "temporal determinations" (zeitliche Bestimmtheiten) of our representations. In this respect, they must be regarded as ensuring the continuity of conscious experience. Temporal structural qualities mediate psychical phenomena that are more complex than non-temporal structural qualities, such as perception of melodies, perception of motion, perception of the appearance and the disappearance of feelings, or that of variations of light and colour. Generally speaking, they are associated with the appreciation of changes, but also with that of duration and the maintenance of representations in consciousness. According to Ehrenfels, psychical phenomena mediated by structural qualities can be themselves compared, and then experienced as relational representations (Relationsvorstellungen) of an abstract nature. Thus, it must be presumed that the first-rate structural qualities (Gestaltqualitäten erster Ordnung) involved in the apprehension of sensory relationships are the substratum of higher-rate structural qualities (Gestaltqualitäten höherer Ordnung). This hierarchical conception of the structural qualities appears very close to that advocated by Fechner about sensations of difference.
1.2. The question of time perception in the 19th century psychology
1.2.1. Perception of successiveness and immediate continuity of conscious experience
The question of time experience became at the beginning of the 19th century a psychological issue of its own. This new epistemological approach of temporality is heavily indebted to German-speaking authors (for a general overview on the issue of time experience in the 19e century, see: Nichols, 1890; Wundt, 1911; Boring, 1950; Fraisse, 1957/1963) and coincides with the rise of a cognitive-like psychological paradigm in German-speaking countries (for the emergence of the cognitive issue in 19th century German psychology, see: Romand, 2010, submitted; Romand and Tchougounnikov, 2009, in press). For 19th century psychologists, time, as a definite phenomenon of conscious experience, is supposed to be explainable in mentalistic terms. Our ability to experience temporal relationships, i.e., basically, to perceive the succession of mental contents in consciousness, appears as a complex cognitive function which should be referred to the activity of simpler qualitatively-defined psychical elements. In this respect, 19th century psychologists openly stand against Kant's conception of time as an a priori form of intuition (Kant, 1968/1965). Time, as they claim, is not an immediately-given and irreducible property of the human mind that should be postulated as a necessary and universal precondition of any objective knowledge. Rather, time perception must be regarded as a product of psychical life itself. The psychologist's task is also to determine the nature of mental processes which contribute to the elaboration of our representation of temporal relationships. However, this does not mean that psychologists try to contest or diminish the importance of time as a cognitive phenomenon. On the contrary, they all insist on the importance of considering experience of temporality of conscious phenomena as an essential condition of the manifestation of conscious experience.
Psychological research on time perception must be replaced in the broader context of the debate about the origin of our representation of extensiveness. For 19th century psychologists and sensory physiologists, space, like time, consists in the manifestation a complex system of relationships between un-extended sensory data. Space is the two- or three-dimensional system defined by the relations of contiguity between sensory data, whereas time is the one-dimensional system defined by the relations of successiveness between sensory data (Boring, 1929; Hatfield, 1990). Psychology of time and psychology of space both address the problematic issue of determining how a representational content of an extensive nature can be built from a whole of merely intensive elements. The question of the origin of time representation is a particularly tricky psychological issue given that our conscious life is by nature fundamentally temporal. Conscious life  consists in a continuous flow of perpetually changing representations: actual experience is renewed at any moment by mental contents which follow up each other in consciousness. Flowing is an essential characteristic of conscious life but cannot be considered as a characteristic proper to mental contents themselves. The succession of representations in consciousness does not correspond to the expression of any particular psychical quality. This is a property that is extrinsic to the entities which compose consciousness. The flow of these entities depends only on the functional dynamics of neural substrates which is itself partially determined by the events of the external world. Temporality of conscious phenomena should nonetheless be experientiable as a psychical phenomenon of its own. But perceiving the succession of representations in consciousness cannot be explained simply by the fact that representations follow up each other. The great merit of 19th century psychologists was to recognize that experience of temporality is not spontaneously given to us with the temporality of conscious phenomena, but ought to be attributed to a specific psychical process. We must be able to represent to ourselves at any time the flow of our own mental phenomena. If not, our conscious life would be hardly more than an inconsistent succession of single experiential moments devoid of any connection with each others. This also raises the difficult issue of determining how mental phenomena which exist at different times in consciousness can nonetheless be apprehended together as a unitary content of consciousness. The various psychological theories of time perception proposed during the 19th century are also as many attempts to solve this apparent paradox.
Until the end of the 19th century, psychologists tried to explain our ability to perceive the flow of conscious phenomena as a process of reconstitution of temporal relationships through an instantaneous, i.e., intensive, act of consciousness. Temporal extension was assumed to be experientiable only indirectly from the reproduction and the comparison of successive mental contents in actual experience. I will study here two models which can be considered as paradigmatic of such a conception of time perception (see also the contributions of Waitz, 1849; Volkmann, 1856, 1876; Brentano, 1874/1973; Lipps, 1883; Mach, 1886/1959; Külpe, 1893; Strong, 1896; Schumann, 1898; Wundt, 1911). On the one hand, I will analyze the model proposed by Herbart in the early 19th century (Herbart, 1816/1964a, 1825/1964b). According to this model, representations which follow up each other in consciousness in a given order can be reproduced (retrieved) in the same order: they are said to build up a definite series. When a series is entirely reproduced, the representations which compose it appear all together in consciousness with an intensity that is inversely proportional to their order of appearance. The gradation of the intensity of the representations generates the feeling of their successiveness. Time experience is conceived by Herbart as an emerging property, since it is not supposed to result from the manifestation of any particular psychical property but simply from the fact that representations are together in a definite relationship in consciousness. On the other hand, I will discuss the theory of the temporal signs which was advocated by Lotze in the second half of the 19th century (Lotze, 1879). As in the previous model, experience of successiveness is supposed to originate in the reproduction of successive representations in the instantaneity of actual experience. But for Lotze, our perception of temporal relationships does not depend on the way representations are reproduced, but on specific psychical factors (the so-called "temporal signs") which indicate what position the latter had respectively in time. In the late 19th century, the conception of time perception based on the notion of the instantaneity of conscious experience was challenged. The postulate of the instantaneity was criticized as being "irrelevant" and "artificial". A new psychological model of time perception emerged, in accordance with the development of the preoccupations regarding the unity and the structure of conscious phenomena, as analyzed in the previous section. Some psychologists also assumed that the succession of mental contents is apprehended intuitively through a temporally-extended psychical act corresponding to the elementary form of conscious experience. Such a conception may be considered somehow as a rehabilitation of Kant's view regarding the spontaneous and original nature of time perception, even if the epistemological perspective remains actually quite different in both cases. I will deal here with James' pioneering ideas on the "specious present" which announce directly the theory of the psychical time of presence that Stern will develop few years later (James, 1890).
1.2.2. Herbart's model of time experience
Johann Friedrich Herbart (1776-1841) is one of the first to have proposed a psychological explanation of the apprehension of temporal relationships (Herbart, 1816/1964a, pp. 386-91; 1825/1964b, pp. 86-112). For Herbart, our ability to experience time results from the elaboration of a "representation of the temporal" (Vorstellung des Zeitlichen). Like the representation of the spatial, the representation of the temporal is a product of our psychical activity: it is elaborated from elementary sensory data, from simple representations that are in themselves devoid of any temporal quality. The representation of the temporal should not be confused with the flow of mental phenomena in consciousness. More precisely, time experience does not consist in the succession of the representations, but in the representation of their succession. This assumption will serve as a guideline for all 19th century theorists of time psychology. Time perception implies the manifestation of a particular act of consciousness that makes us capable of apprehending mental phenomena that flow during a short interval of time. But this does not mean that Herbart considers the representation of the temporal as a temporally-extended act of consciousness. The extensive character of what is represented (das Vorgestellte), as he underlines, does not imply that the fact of representing (das Vorstellen) is itself an extensive phenomenon. Our representation of the temporal, as any other act of consciousness, is for Herbart a phenomenon of an intensive nature. Temporal relationships are therefore apprehended instantaneously in the unity of the soul. According to Herbart, the representation of the temporal originates in the reproduction of series of representations (fig. 1). Simple representations that appear successively in consciousness can be associated and maintained together in a definite order: they form particular series (Reihen). When the member of a series is recalled into consciousness, the other members tend to be recalled together with it. The series is then said to be reproduced. The reproduction appears as a means of restituting the order of appearance of representations in consciousness. Once reproduced the first member of a series, the other members are reproduced successively according to their respective position in the series. When the last member is reproduced, representations that compose the series are present altogether in consciousness, fused in a single intensive psychical unit. Within this representational complex, each representation has a particular intensity that decreases all the more so as the representation was reproduced earlier. The reproduced series is then experienced as a graded psychical phenomenon in which the representations that reappeared last stand out against a vague background formed by those that reappeared first. In that case, representations seem to be more or less remote or more or less close, as they stay more or less behind or ahead of the series. The representational complex is interpreted as a system of temporal relationships: it constitutes what Herbart calls the representation of the temporal.
- Figure 1: Herbart's model of time perception
1.2.3. Lotze and the theory of the temporal signs
Hermann Lotze's theory of the temporal signs (Temporalzeichen) is another attempt to explain psychologically our experience of time (Lotze, 1879, pp. 268-302). To some extent, the argument defended by Lotze (1817-1881) is close to Herbart's. Like Herbart, Lotze considers that there is no primary perception of temporal relationships: time apprehension (Zeitauffassung) - that he also calls "time representation" (Zeitvorstellung) - must be regarded as a product of the psychical activity itself. According to Lotze, the origin of our experience of time lies "only in the very content of the [psychical] events, not in a form existing outside of it in which it would fall (...)" (Lotze, 1879). The act of apprehending the temporal relationships is regarded by Lotze as having the same character of immediacy as any other mental phenomenon. Also on this issue Lotze agrees with Herbart: the temporal order of psychical contents is represented (vorgestellt) through an instantaneous act of consciousness. Both of them maintain that representations perceived as being successive are in reality simultaneous mental events. But unlike Herbart, Lotze considers that the perception of succession results from an act of comparison between the representations. According to him, it is only by comparing two contents that we are able to decide which one occurred first and which one occurred last. Thus, the question is to determine how representations that occur simultaneously in consciousness can nonetheless be perceived as having in fact occurred in a definite temporal order. In these circumstances, how to distinguish what is experienced as being present (das erlebte Anwesende) from what is represented as being absent (das vorgestellte Abwesende)? In other words, how to know at a given time that something actually exists and that something else does not exist anymore in our consciousness? For Lotze, there should be "qualitative differences" by virtue of which the representations are attributed to a well-defined place in the flow of conscious events (fig. 2). Lotze calls these putative psychical entities "temporal signs" (Temporalzeichen), by analogy with the "local signs" (Localzeichen), the mental properties which confer according to him their spatial character to visual and tactual sensations (Lotze, 1879).
- Figure 2: Lotze's model of time perceptionts
1.2.4. Time perception as a temporally-extended psychical phenomenon: James' "specious present"
One cannot conclude the survey on the question of time experience in the 19th century without mentioning the contribution of William James (1842-1910). James' most complete investigation on perception of time is developed in the Chapter XV of The Principles of Psychology (James, 1890, pp. 605-42). For James, our knowledge of time-relations cannot be explained from the simple fact that mental contents flow in consciousness. The fact that mental contents succeed each other does not imply, as such, that we are capable of apprehending them as temporally distinct psychical events. Perception of time, as James insists, must be regarded as a psychological problem of its own:
"A succession of feelings, in and of itself, is not a feeling of succession. And since, to our successive feelings, a feeling of their own succession is added, that must be treated as an additional fact requiring its own special elucidation (...)."
The argument developed by James in this famous excerpt from the Principles has been in reality previously discussed by a number of German psychologists. As seen above, it is already encountered in Herbart's works in the beginning of the 19th century. Here, James refers directly to Volkmann's investigations (Volkmann, 1876); more generally speaking, he willingly admits being deeply indebted to the German research tradition on perception of time. James' originality lies rather in the fact that he is probably the first to advocate explicitly that experience of time is an experience in time; in other words, a temporally-extended psychical phenomenon. According to James, indeed, what we are aware of is not what is instantaneously present in consciousness, but a certain stretch of the stream of consciousness . Besides the actual present of conscious phenomena, there is a specious present that corresponds to "the short duration of which we are immediately and incessantly sensible" (James, 1890, p. 631). The specious present consists, in other words, in apprehending a set of successive mental contents as one single psychical unit:
"We do nor first feel one end and then feel the other after it, and from the perception of the succession infer an interval of time between, but we seem to feel the interval of time as a whole, with its two ends embedded in it. The experience is from the outset a synthetic datum, not a simple one; and to sensible perception its elements are inseparable (...)."
The specious present can be defined as the time in which we are capable of experiencing intuitively, that is to say, spontaneously and independently from any kind of reasoning, mental phenomena that flow in consciousness. For James, the specious present is not a clear-cut segment of consciousness, but a psychical phenomenon "that has a vaguely vanishing backward and forward fringe" (James, 1890, p. 613). If it is therefore difficult to estimate accurately its duration, one can estimate on the basis of a number of experimental results that it does not exceed a dozen seconds. James compares the specious present to "the rainbow on the waterfall, with its own quality unchanged by the events that stream through it" (James, 1890, p. 630): its content is an incessant turnover of mental phenomena whose temporal value changes at any moment. Every manifestation of the specious present encompasses a certain space of time that has just elapsed. In this respect, the specious present appears to be the basis of primary or elementary memory, i.e., the ability to retain the immediate past without reproducing previously experienced representations . It constitutes "the original paragon and prototype of all perceived times" (James, 1890, p. 631) from which we can infer more complex time relationships, such as the date or the duration of conscious phenomena. For James indeed, the relationships of this nature are not intuited but symbolically apprehended from representations that are reproduced in the specious present.
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