The tendency to categorize the world into 'similar' and 'different' is fundamental to human cognition. It underlies folk knowledge systems in orally-reliant societies as well as sophisticated taxonomies in science (Durkheim 1912; Levi-Strauss 1962; Smith and Medin 1981; Sokal 1985; Berlin 1992). It is shared to some degree even by species far remote from us on the phylo-genetic scale (Giurfa et al. 1996). The term 'taxonomy' itself is, however, a newcomer to language: taxinomie,or taxonomie, was introduced in 1813 into French to denote classification of entities, and the discipline that deals with such classifications (from taxis + nomie, meaning arrangement + method in Greek; Le Maxidico 1996). It then found its way into English. In biology, 'taxonomy' came to be associated predominantly with the discipline that classifies biological species (Mayr 1981). But the term is also widely used in other domains of knowledge to denote classifications of a variety of natural kinds, concepts, and artefacts.
A major issue concerning taxonomies of natural entities is whether they represent 'natural' types or only creations of the human mind. Some claim that 'natural' reflects the state of affairs in nature; others that it only reflects the capacities of the human mind; and still others that both the above coincide, as the human mind is expected to have evolved to equate 'natural' in nature with 'natural' in mind (Sokal 1985). A potential clash of 'natural in mind' and 'natural in nature' is illustrated in the taxonomy of biological species: early schools of taxonomy relied on the phenotypic similarity of organisms and therefore shared much with primitive taxonomies; later taxonomies already rely on the more refined scientific understanding of phylogenesis (Mayr 1981; Sokal 1985). Certain classifications do appear intuitively 'unnatural'. Consider, for example, the fictitious taxonomy attributed by Borges (1952) to a Chinese encyclopaedia, Celestial emporium of benevolent knowledge. According to this, animals are divided into: '(a) those that belong to the Emperor,
(b) embalmed ones, (c) those that are trained, (d) suckling pigs, (e) mermaids, (f) fabulous ones, (g) stray dogs, (h) those that are included in this classification, (i) those that tremble as if they were mad, (j) innumerable ones, (k) those drawn with a very fine camel's hair brush, (l) others, (m) those that have just broken a flower vase, (n) those that resemble flies from a distance'. The beauty of this taxonomy is in its poetic oddity ("surprise). However, as classifications are intended to facilitate handling and analysis of information, even the aforementioned emporium could have made some sense from the point of view of a ruling Emperor. Further, we should not forget that to the modern mind, 'primitive' classifications of natural phenomena may appear rather confused, even though they were considered perfectly logical to the contemporary mind (Hallpike 1979). For example, the Greek world was depicted as composed of four basic elements, water, air, fire and earth (Plato, Timaeus 32b-c); hence taxonomies are "culture dependent and what is regarded as 'natural' in 2001 may not be so in 2100.
Taxonomies prevail at every branch of knowledge and "level of analysis. Examples in the neurosciences are types of neurons, glia, "ion channels, "receptors, "neurotransmitters, "intracellular signal transduction cascades, neuronal firing patterns, brain regions and pathways, etc. These 'types' contain multiple 'tokens', i.e. specimens or instances of the type (Dudai 1993; "system). The taxonomy most characteristic of memory research is no doubt that of memory itself. 'Memory system' could be described as an organized structure of interconnected neural substrates, encoding experience-dependent "representations that subserve some characteristic type(s) of behavioural and cognitive function(s) (Tulving 1985; but see below). Each system could hence be specified by phenotypic, functional, structural, and possibly phylogenetic "criteria (e.g. Tolman 1949; Sherry and Schacter 1987; Shettleworth 1993; Schacter and Tulving 1994).
The mere notion that memory is not monolithic pre-dates science. Over the years, multiple axes or "dimensions have been used to classify memory. For example, sensory modality. 'In memory, all things are kept distinct and according to kind. Each is brought in through its own proper entrance. the light and all the colors ... all these enter in, each by its own gateway, and are laid away within it.' (Augustine 400). Or duration, i.e. whether short or long lived (James 1890; Hebb 1949; "phase). Or actualization, i.e. whether active or inactive at a given point in time (Lewis 1979; "retrieval). Or the processes that drive the system, either top-down, concept-driven, or bottom-up, data-driven
(Roediger 1990).1 Additional candidate dimensions are illustrated in "collective memory, "dimension, "learning, "prospective memory. Most taxonomies of memory are not mutually exclusive; one could consider a short-term, visual memory, or an inactive, long-term memory.
A cardinal "criterion in the prevailing taxonomy of long-term memory is that of "conscious awareness, i.e. whether the information is accessible to conscious recollection or not. This type of dichotomy was imported into modern neuroscience from philosophy. Kant (1781) distinguished representations with or without consciousness; de Biran (1804) spoke of'mechanical memory', in which recall is a 'simple repetition of movements', and 'representative memory', in which recall involves 'the clear appearance of . (an) idea'; Bergson (1908) differentiated between habit, the 'memory that repeats', and 'memory per excellence', or 'the memory that imagines'; and Ryle (1949) distinguished 'knowing how' from 'knowing that'. 'Knowing how' is also known as 'practical knowledge', and 'knowing that' as 'propositional' or 'factual knowledge' (Bernecker and Dretske 2000).
The 'that' vs. 'how' dichotomy is supported by analysis of memory deficits in human "amnesics and to a certain degree by animal "models of amnesia (Mishkin et al. 1984; Squire and Zola 1996; Eichenbaum 1997a). Accordingly, the current "zeitgeist taxonomy of memory systems in the brain sciences depicts two metasystems of long-term memory, "declarative (the 'that' system, alias 'explicit') and nondeclarative (the 'how' system, alias 'implicit') (Schacter 1987; Squire and Zola 1996; Figure 63). Declarative memory is further divided into memory for facts ('semantic') and for events ('episodic', 'autobiographical', Tulving 1983). Some authors draw a distinction between 'episodic' and 'declarative' (Tulving and Markowitsch 1998). Similarly, in epistemology, 'knowledge by acquaintance', i.e. of people, places, and things, is distinguished from propositional or factual knowledge (Bernecker and Dretske 2000).
Declarative memory is subserved by diencephalic and mediotemporal structures ("cerebral cortex, "hippocampus, "limbic system). Some authors propose that brain systems that subserve "episodic and semantic memory are partially dissociable, with only episodic memory being fully dependent on the hippocampus (Vargha-Khadem et al. 1997; see Tulving and Markowitsch 1998 for support of this view, and Squire and Zola 1998 for a different position). Nondeclarative memory is commonly further subdivided into the following systems (Figure 63). (a) non"associative
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