[Journal für Philosophie & Psychiatrie, Juli 2011, Supplement]
2. Commentary of Stern's 1897 paper on the "psychical time of presence"
2.1. The irrelevance of "the dogma of instantaneity"
Stern considers unsatisfying the various theories of perception of time proposed until then. According to him, psychologists have been completely misled by "the dogma of instantaneity" (das Dogma der Momentaneität): they have believed, wrongly, that temporal extensiveness should be appreciated through an infinitesimally short act of perception (Stern, 1897a, pp. 325-26). For Stern, it is nonsensical to assert that one can experience temporal relationships between mental contents that are simultaneously present in the unity of consciousness. Generally speaking, Stern challenges the opinion that conscious experience necessarily occurs during an instantaneous psychical act. According to him, nothing is more artificial than isolating at a given moment a section (Querschnitt) of psychical life, because in such case one sets aside mental elements which are not actually separate in consciousness (Stern, 1897a, p. 326). The representation of a polysyllabic word is a good example of psychical entity which cannot be contained within an immediate act of consciousness. Stern argues that in this case it is hard to consider the representations of syllables as separate conscious phenomena, but that it is also hard to consider them as elements absolutely simultaneous in consciousness. It is the same case for the letters perceived within a word or a syllable: although the representations of letters take part in one single psychical entity, they must also be consecutive in a well-defined order. Stern suggests here that we consider the monosyllabic word "rot" (red): if the three representations of "o", "r", and "t" were simultaneous, then it would be impossible to distinguish the word "rot" from the words "Ort" (place) and "Tor" (gate). Without an actual succession of the representations of letters and syllables, so without certain duration of the word representations, words themselves would be thoroughly meaningless (Stern, 1897a, p. 328). For Stern, it must be admitted that the succession of representations in consciousness is apprehended directly (unmittelbar) within a short interval of time (Zeitstrecke) (Stern, 1897a, p. 334). Psychologists have considered up to now that the succession of representations could be perceived only indirectly, by means of "indications" (Indizen) which replace the actual temporal order of representations in a given point of time (Stern, 1897a, pp. 331-32). In Stern's opinion, this argument is completely inconsistent with the introspective analysis. Stern does not dispute the fact that one may be aware of temporality through an instantaneous act of consciousness. But according to him what one may be aware of instantaneously is just an abstract representation of time, and not the flow of the mental contents. The direct awareness of time (die direkte Zeitanschauung) is itself only possible during an act of consciousness of certain duration (Stern, 1897a, pp. 331 and 334).
2.2. The subjective present is mediated by a temporally-extended act of consciousness
For Stern, the apprehension of temporal relationships depends on a temporally extended content of consciousness (ein zeitlich ausgedehntes Bewusstseinsinhalt). Such content must be regarded as an indissoluble grouping of successive psychical events: it forms an independent psychical unit, as any immediate act of consciousness (Stern, 1897a, p. 331-33). As Stern emphasizes:
"The psychical event that occurs within a definite temporal interval can build in some circumstances a coherent and homogeneous act of consciousness, notwithstanding the non-simultaneity of the single parts. - I call „time of presence' the temporal interval in which such a psychical act is likely to extend." 
According to Stern, successive representations constitute one single content of consciousness insofar as they occur within the same "time of presence" - or more exactly within the same "psychical time of presence" (psychische Präsenzzeit) (fig. 3). The psychical time of presence is a segment of consciousness which enables the grouping, and then the experiencing of mental contents that are put in order in time: it is the act of apprehension (Auffassungsakt) by means of which the temporal relationships can be perceived (Stern 1897a, p. 329-30). But in this instance, Stern observes:
"We do not only perceive those temporal relationships, but also the time of presence itself, or, more exactly speaking, the temporal presence itself, i.e., 'its content seems to us as being present'." 
The mental contents which follow each other in the time of presence all participate of the same temporal unit: the temporal flow is experienced as something present (gegenwärtig) (fig. 3). For Stern, the present (die Gegenwart):
„(...) can be defined as the embodiment of the spatiotemporal relationships that can be the object of a direct perception. When we become aware of the immediacy of perception, then the object of the latter appears as „present' to us." 
- Figure 3: Stern's general model of perception of the present
From a psychological point of view, the present is then nothing but what we are directly aware of. But Stern's present is by no means punctual: it corresponds to the manifestation of an extended psychical act and to the apprehension of an extensive mental content. According to Stern, there is a strict analogy between both temporal and spatial apprehensions. As the temporal relationships are apprehended within a time of presence, the spatial relationships are apprehended within a "space of presence" (Präsenzraum): they are experienced as "being here", as the temporal relationships are experienced as "being now". In short, there are for Stern two kinds of present: the temporal present and the spatial present. There is a "before" and an "after" in the temporal present, as there is a "right" and a "left", a "closer" and a "farther" in the spatial one. But there is no "past" and no "future" in the temporal present, neither there is any "absence", any "over there" in the spatial present (Stern, 1897a, p. 333). "Past", "future", "absence" are spatiotemporal relationships which can only be reconstructed (erschliessen) by inference. According to Stern, a large variety of temporal relationships can be nonetheless directly apprehended within the time of presence:
"(...) the temporal relationships encompassed in the time of presence are the object of the apprehension: they correspond to this category of psychical entities that are baptized under the name of the „sense of time' or the „immediate consciousness of time' (Meumann). Duration, succession, rhythmical structure, velocity, acceleration, can be perceived directly, the temporal intervals can be directly compared one to the other. But this is possible only if the totality of the material that is perceived or compared is encompassed in the psychical time of presence" 
The hypothesis of the psychical time of presence not only enables us to explain the perception of succession, but also that of numerous other temporal characteristics. In Stern's opinion, it permits to explain in a simple manner the various aspects of the sense of time: the nature of the temporal experience depends on the nature of the temporal relationships, and not on the way of apprehending them. This or that other kind of temporal relationship is perceived as soon as it falls into the time of presence.
2.3. The phenomenon of the "projection" into the time of presence
Past and future are what Stern terms "abstract representations of time" (Stern, 1897a, p. 340). Indeed, according to him:
"(...) temporal relations that are experienced as being past or future can be represented only indirectly, schematically, symbolically, and only because we try to transfer them [into the time of presence] by enlarging their proportions. In that case, intuitiveness is lost." 
We only have an indirect awareness of past and future: our representation of past and future events is nothing but the symbolic manifestation of an actual experience (Stern, 1897a, p. 340) . Nevertheless, the abstract representations of time should be experienced (erlebt) in some way. For indeed we do not only represent to ourselves the events as being past or future, we also perceive them as a temporal flow. In other words, our symbolical temporal representations must themselves be immediately perceptible (anschaulich). Now, it has been seen that the temporal relationships are immediately perceived as far as they are apprehended in the time of presence. So, for Stern, the abstract representations of time can be experienced only by means of a projection into the time of presence (Projektion in der Präsenzzeit) (Stern, 1897a, pp. 334-337) (fig. 4). The temporal relationships "projected" (projiziert) into the time of presence have the characteristic of being much shorter than they were before or will be in reality. In order to form a single coherent act of consciousness, the past or future perceptual episodes must be represented on a smaller scale. Then, the projection in the time of presence corresponds to a phenomenon of compaction of the temporal succession (Stern, 1897a, pp. 335-36). An analogous phenomenon is noticed in the case of space experience. Some spatial relationships, as geographical tracts, cannot be directly perceived as a whole. Nevertheless, one only needs to represent them graphically in a smaller scale to apprehend them all at a glance (Stern, 1897a, pp. 334-35). The projection into the time of presence results in a genuine perceptual experience (Anschauung) of spatiotemporal relationships. Stern provides us with several examples. When one is singing, he/she does not only perceive the bar being sung, but already catches a glimpse of the next bar. This anticipated percept is not a jumble of simultaneous tones: the temporal relationships between the pitches are preserved, although the absolute tempo is considerably accelerated. A comparable phenomenon of perceptual anticipation can be observed in the case of blind people: when reading, blind people always touch the word next to the one they are actually reading, in order to get a schematic image of it (Stern, 1897a, p. 335). Dreams are another example of temporal contraction of perceptual data. The dream experience usually seems to last for hours, whereas in reality it lasts few minutes at most. For Stern, all the temporal relationships constituting a dream can be collected in the time of presence (Stern, 1897a, p. 336). The elaboration of voluntary actions also depends on a phenomenon of projection into the time of presence. Stern states here that we represent to ourselves all the progress of the action to be performed as a shortened representational content (Stern, 1897a, pp. 335-36).
- Figure 4: Stern's theory of «projection»
2.4. Time of presence and memory
Stern, as many psychologists of his time, considers that the continuity of conscious experience through time is determined by the comparison of successive mental contents (Stern, 1897a, pp. 328-29). The consistence of our psychical phenomena depends on our capacity of judging the identity, the equality, the resemblance, or the difference between what is occurring and what occurred in consciousness. In particular, sensory data perceived at a given moment would be thoroughly meaningless if they could not be related to those perceived immediately before (Stern, 1897a, pp. 337-39). For Stern's contemporaries, the psychical continuity between what is instantaneously present and what has just elapsed is ensured by the primary memory (primäres Gedächtnis). As Stern recalls, psychologists term "primary memory" the fact of retaining for a very short time an image of the representations after their disappearing from consciousness . Primary memory is supposed to be an autonomous form of memory, qualitatively different from the regular memory. The primary memory images (primäre Gedächtnisbilder) would be then characterized by a particular vivacity and an immediate perceptibility, likely to confer a particular force to the mnemonic judgment. So, they would be closer to the actual representations than to the genuine memory images (Stern, 1897a, p. 338). In reality, primary memory would consist in the persistence, and not in the reappearance, of representations in consciousness (Stern, 1897a, pp; 338-39). According to the accredited theory of the primary memory, the coexistence of the actual representations and of the primary memory images permits to compare at any moment what is being perceived with what has just been perceived. For Stern, such an explanation is unacceptable. Psychologists are misled in believing that the simultaneity of mental contents is a prerequisite to their comparison, and that one can apprehend what has elapsed only indirectly through an instantaneous act of consciousness (Stern, 1897a, p. 337-38). On the contrary, Stern argues that the temporal continuity of conscious experience results from the direct successive comparison of the mental contents: as soon as they are contained in the same time of presence, the successive mental contents are immediately related to each other. The psychical time of presence permits to retain for a short time the representations in consciousness (Stern, 1897a, p. 339). In other words, it ensures the short-term memorization of the sensory data. The hypothesis of the psychical time of presence allows us to explain the primary memory phenomenon without referring to the notion of the "memory image" (Stern, 1897a, p. 337).
But it does not mean that for Stern the memory phenomenon is reduced to a direct successive comparison of the mental contents. If primary memory is the capacity of experiencing what belongs to the past as something actually present, regular (or secondary) memory corresponds on the contrary to the capacity of experiencing as a part of the past what is actually present in consciousness. Remembering something is in reality nothing but to apprehend a representation with the feeling of having already apprehended it. For Stern, as for some other psychologists of his time, the secondary memory phenomenon is explainable by the manifestation of a "quality of familiarity" (Bekanntsheitsqualität): there must be a category of mental entity likely to confer an impression of déjà-vu to the sensory data which occur at the same time in consciousness (fig. 5) . Stern states that, without the existence of such a psychical factor, the so-called memory images would be totally undistinguishable from the actually-experienced representations (Stern, 1897a, pp. 339-40). The quality of familiarity is a symbol (Symbol) by means of which we interpret temporal relationships that are projected in the time of presence as having already been experienced before. Secondary memory permits to compare indirectly past perceptual episodes that are symbolically represented in the time of presence with the mental contents that are actually perceived: it ensures in this respect the long-term continuity of conscious experience. The continuity of conscious experience implies in reality the ability of comparing actual mental contents, not only with past, but also with future mental contents. For Stern, as mentioned above, the future, like the past, can be experienced either directly (immediate future) or symbolically within the psychical time of presence. Stern also admits without really admitting it explicitly the existence of a primary and of a secondary memory of the future.
- Figure 5: the process of secondary memory according to Stern
2.5. The duration of the time of presence
In Stern's opinion, the time of presence should be described in quantitative terms, and not only in qualitative ones. The time of presence has been defined as a psychical phenomenon that is temporally-extended; thus, it should be possible, at least theoretically, to determine its duration (Dauer) (Stern, 1897a, p. 340). As Stern recalls, the duration of the time of presence is surely not identical to the duration of a representation. The time of appearance of a mental content in consciousness is not the time required for its apprehension through a coherent act of consciousness. A representation may often continue far beyond the scope of the psychical time of presence (Stern, 1897a, p. 341). "The projection into the time of presence" is a source of additional difficulties. As mentioned before, the temporal sequences projected in the time of presence are more or less shortened so that they can be apprehended as one and unique psychical content (fig. 4). In that case, the experienced duration of the projected sequences may be very different from the effective duration of the corresponding time of presence. In short, one must be particularly cautious while trying to calculate the duration of the psychical time of presence (Stern, 1897a, p. 341). In this respect, Stern criticizes James' results about the duration of the "specious present". These results have been obtained from studies on the perception of tone series. According to Stern, the values obtained simply indicate the objective duration of the perceptual complexes, and not, as James pretends, the duration of the psychical present. Stern criticizes James in particular for having disregarded the phenomena of projective recapitulation of the mental contents, and therefore for having largely overestimated the duration of the specious present (Stern, 1897a, p. 342).
For Stern, the psychical time of presence has no standard duration: it may appear instantaneously or last for several seconds; any intermediate value is possible. The duration of the psychical time depends on several factors: the qualitative and quantitative nature of its content, the direction of the apprehension, and what Stern calls rather mysteriously "the force of the psychical energy" (Stern, 1897a, p. 342). Furthermore, Stern considers that no maximal value can be attributed to the time of presence. For, in that case, one should determine the length of an act of consciousness with particularly uncertain limits, and whose ratio of temporal projection is hardly estimable (Stern, 1897a, p. 342). If we cannot calculate the maximal value of the time of presence, it is still possible to determine its optimal value (Optimalwert). The optimal value of the time of presence is defined by Stern as the "culmination point" (Kulminationspunkt) specific to every temporally-extended act of consciousness (Stern, 1897a, pp. 342-43). In fact, it may be characterized in two different ways. On the one hand, the optimal value is the value of the psychical time of presence that the subject considers as "the most pleasant", i.e., the duration he/she implicitly recognizes as the most suitable for apprehending a given psychical content. Stern talks here about "adequate time" (adäquate Zeit). On the other hand, the optimal value corresponds to the duration required for the full unfolding of a consistent act of consciousness, irrespective of how this duration is appraised subjectively: it is "the most favorable time" (die günstigste Zeit) (Stern, 1897a, p. 347). According to Stern, the existence of optimal durations of apprehension is evidenced by a number of studies on auditory perception. Adequate times can be established for the listening of melodies, rhythms, or language. The adequate time of a melody is defined with respect to a certain tempo: for each kind of melody, there is a speed at which the tone succession is apprehended most easily and felt as the most agreeable. With regard to rhythms, we notice on the contrary that the adequate time tends to remain the same (about one second) whatever the rhythmical structure may be. It has been shown that the quicker the time beats are, the more numerous they become (Stern, 1897a, p. 345-46). In the case of language, the adequate time may have very different values and depends on one's ability to appreciate the pleasantness of a speech without hastening and without artificially hampering the train of one's thoughts. The time of a language that one does not understand is typically inadequate: in that case, the speech gives the impression of flowing too fast. The linguistic adequate time is lengthened as soon as listening to a speech requires a significant effort of concentration and reflection (Stern, 1897a, pp. 346-47).
Stern's thoughts on the duration of the time of presence directly echo the studies on the "time sense" (Zeitsinn), the experimental program of research on time perception initiated by the German physiologist Johann Nepomuk Czermak (1826-1873) in the mid-19th century (Czermak, 1857/1873). In his seminal 1857 paper, Czermak sketched the outlines of a "physiological experimental investigation on sensory perception of velocities, considered in general" (Czermak, 1857/1873, p. 417), i.e., of a scientific exploration of the "time sense", based on the model of the famous research carried out some years before by Ernst Heinrich Weber (1795-1878) on the "space sense" (Weber, 1846). His aim was to determine the physiological conditions of the perception of objective temporal relationships, without worrying about the nature of the fundamental psychical mechanisms that mediate the appearance of our representation of time. As a matter of fact, studies on the Zeitsinn have been developed in the second half of the 19th century as a program of research distinct from theoretical psychological studies on time experience. They relate to the quantitative aspects relating to the flow of mental phenomena in consciousness such as tempo, rhythm, duration, etc., that is to say, all what corresponds to what psychologists term nowadays "timing perception". The Zeitsinn program of research was very successful in Germany in the second half of the 19th century and early 20th century (Vierordt, 1868; Meumann, 1892-93; Wundt, 1911; Benussi, 1913), and has been pursued and developed uninterruptedly until now (Durup and Fessard, 1930; Rubin, 1949; Fraisse, 1957/1963, 1974; Vicario, 1973; Michon, 1978; Patel, 2003; Thaut, 2003; Meck, 2005). The lexicon and the concepts ("adequate time", "most favorable time", "optimal value"), as well as the examples (perception of melodies, rhythms, language) used by Stern are typically inspired from contemporary studies on the Zeitsinn. In this respect, one may wonder to which extend the analysis he suggests is really relevant to understand the issue of the duration of the time of presence.
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