Owing to both historical and immanent factors, the most popular and widespread approach to classifying views on the relation between the soul and the body is that which emphasizes the problem of substances as the basis of psychic and physical life. This is a question of great metaphysical importance, taken up by the authors of the 17th century in their world views: Descartes's dualism, Spinosa's monism, Hobbes's materialism, Leibniz's spiritualism. Its role was also stressed by Wundt, who held that there were three fundamental notions where metaphysics has long sought a definition: These are the notions of substance, matter, and the soul [Wundt 1909, 348f] - these being prominent, since all other general principles depend in one way or another on these three concepts. Therefore, they are the primary features of any world view. And although Wundt's task is primarily a classification of world views, it is a classification of views on the relation between the soul and the body as well. For Wundt, problems concerning the relation between the soul and the body have given rise to conflicting hypotheses about substance. [Wundt 1911, 738]. This is not strange, in the light of Becher's remark that solving the relation between the soul and the body is central to the problematic of world view. [Becher 1911, 329].
   The prominence of substance in the classification of views on the relation between the soul and the body, is most clearly reflected in its provision of the principle of classification. Höffding, for instance, claims that, as far as the relation between the soul and the body, between consciousness and the brain, is concerned, "there are... one dualistic and three monistic hypotheses possible". The former assumes that the body and the soul affect each other as "two different beings or substances". On the other hand, according to the first monistic hypothesis, "the soul is an aspect or product of the body". The second contends that "the body is an aspect or product of one or more psychic beings". The third claims in turn that the soul and the body, consciousness and the brain, develop "as concurrent aspects of the manifestation of one and the same being". [Höffding 1887, ch. II. Seele und Körper]. A similar classification of views is to be found in Eisler's philosophical dictionary, in which he cites dualism first, according to which the soul and the body are two substances or two kinds of processes. This he juxtaposes with the monism that finds the soul to be the essence of a thing, whose manifestation is the body; this he calls spiritualism. If, on the other hand, one claims that the soul is a manifestation or function of the body, this is materialism. A monism which believes the soul and the body to be two manifestations, two modes of existence in the same being, is a theory of identity. [Eisler 1910, 1266].
   Bain's classification is identical with Eisler's at the outset, though it diverges further on. Bain divides views on the relation between the soul and the body into two main categories, according to whether they assume one or two substances to be the ultimate elements of human being. [Bain 1874, 170f]. Again, notions of monism and dualism are emphasized.
   Unfortunately, classifications that reduce views on the relation between the soul and the body to monism or dualism make use of terms which are largely underdetermined. And so materialism is usually numbered among monistic views, as it recognizes only one kind of substance, namely matter. Yet recognizing only one kind of substance may well be consistent with presuming two kinds thereof. A well known example of such a view is ancient materialism. Although it rejects spiritual substance as a separate and distinct basis of mental life, it recognizes a separate kind of atoms constituting the soul, differing from those which constitute the body. This view has been customarily labelled dualistic materialism. It must be noted, however, that the sense of "dualistic" here differs from that which it acquires when we refer to Descartes' view. In Descartes' sense, dualism refers to two different kinds of substance, whereas in ancient materialism it refers to two different types of substance. Thus, a terminological confusion arises whereby materialism is numbered among monistic views, despite the existence of terms such as "dualistic materialism". Similarly, one could also speak of dualistic spiritualism though, like materialism, spiritualism is classified as monistic. Leibniz's spiritualism could be termed dualistic, since it presumes differing numerical principles for the soul and the body. The soul is always one monad, whereas each body is constituted in a combination of monads. A certain duality or juxtaposition can be observed here.
   Yet more important than the wavering sense of dualism is another terminological shakiness, namely that concerning monism. This term combines two clearly different senses: at times, monism connotes a view which recognizes only one kind of substance at other times, however, it describes a view which recognizes only one substance. In the first sense, materialism and spiritualism are labelled monistic, since each reduces everything to only one kind of substance, physical or spiritual. In the second sense, on the other hand, Spinosa's view is monistic in that it reduces everything to one and only one substance. The former is a monism of kind, and is qualitative, whereas the latter is a quantitative, numerical monism. The homogeneity of substance is at issue in the first case, no matter how many substances there are; in the second case, it is the uniqueness of substance that matters, regardless of its kind.
   Philosophers have sought to right this terminological shakiness, both by introducing new terms and by defining existing terms more precisely. Separate terms for a monism of kind and for numerical monism were necessary. The word "singularism" seemed particularly apt the only problem being to decide which monism should assume it, leaving the term "monism" to the other.
K,lpe speaks of six different points of view used in the classification of outlooks. The first concerns "how many principles should be assumed for discriminating world views: possible views in this matter are usually classified under the headings "monism", "dualism", and "pluralism". Yet the first two names usually express qualitative differences; singularism would be a better match for pluralism, as a purely quantitative opposite". [Külpe 1895 - Twardowski refers here to the 4th edition, p. 119 (ed. note)]. Thus it would seem that Külpe wishes to rename quantitative monism "singularism", reserving the word "monism" for qualitative monism. Our surmise is further confirmed by one more passage on the same page; in speaking of the second point of view, he names the qualitative principles as its characteristic feature. Here he again places monism in the context of other approaches [Ibid.].
   However, [later] Külpe writes again that "dualists who, like Descartes, regard the spiritual and the material factors as two real beings, different as to their essence, may be considered pluralists. On the other hand materialists, spiritualists, and monists may be regarded singularists. Thus a spiritualist like Leibniz, who ascribes to all his monads an essence homogenous in terms of quality, must be numbered among singularists" [Twardowski refers here to the 4th edition, p. 170f (ed. note)]. From these words it follows unambiguously that singularism is to refer to a monism of kind rather than to a numerical monism, since in numerical terms Leibniz is a pluralist. In other words, it is K,lpe himself who sometimes uses the word "singularism" in the sense of a numerical monism, and sometimes as a monism of kind. In doing so, he may contribute more to the confusion of terms, than to their disambiguation.
   The disambiguation of terminology will probably follow the example set by Külpe's first statement: That is, the name "singularism" will refer to numerical monism, whereas the name "monism" pur et simple will be reserved for a monism of kind, unless it is named unism or unitarism.
   Yet this classification is also deficient, for another reason. It was drawn up at a time when views denying the existence of substance, especially spiritual substance, were but incipient (Hume retraction) and were not as significant or influential as they were later to become. However, once they attained a position on a par with views recognizing the existence of substance, any classification of views on the relation between the soul and the body which is based on how many (or how many different) substances are assumed to constitute the background of mental and physical life must prove insufficient.
   It has been necessary to find a way of extending this classification so as to include so-called actuality theories, according to which the soul is a tissue of mental functions or acts side by side with substantialist theories, which either see the soul as a separate substance or reduce it to substances of some other kind. This has been achieved by once more splitting the sense of the words "monism" and "dualism" so as to embrace both substantialist and actuality theories. As far as the substantialist theories are concerned, these words have retained their previous sense; for actuality theories, "monism" and "dualism" have taken on a new sense. Monism, for instance, refers, not to a view that assumes one and only one substance or only one kind thereof, but a view which takes mental and physical acts to be homologous or even identical facts or processes. This is how notions of substantial and phenomenal monism and dualism have come into being. [Höfler 1897, 49f]. Substantialism is not even the only possible non-phenomenalistic view, since one can assume a standpoint which reaches beyond phenomenalism without adhering to a substantialist theory. Such a position is assumed by those who claim that the soul and the body are only two sides of one and the same actual course of facts. Thus phenomenalist monism and dualism must be juxtaposed with metaphysical monism and dualism, with substantial monism and dualism being only special cases thereof. [Ibid, 50].
   A view on the relation between the body and the soul may be dualistic from the phenomenalist point of view, while being monistic in metaphysical terms, as is, for instance, spiritualism or the parallelism cited earlier. In addition, factors of kind and number come into play. Thus, the words "monism" and "dualism" appear in four different senses: monism and dualism of kind, of number, metaphysical or phenomenalist. While describing a view as monistic or dualistic, one must clarify at the same time the sense used in each case. It is not enough, then, to call Leibniz's spiritualism a metaphysical monism and a phenomenalist dualism; it should be added that it is a monism of kind.
   This example proves also that a distinction between numerical monism and dualism, as well as between monism and dualism of kind, applies in a natural way only to substantialist, or more precisely metaphysical, monism and dualism not to phenomenalistic monism and dualism, nor to metaphysical, non-substantial monistic or dualistic theories.

[Translated by Zofia Kolbuszewska]

[Bain 1874] A. Bain, Geist und Körper. Die Theorien ,ber ihre gegenseitigen Beziehungen, Leipzig, Brockhaus.
[Becher 1911] E. Becher, Gehirn und Seele, Heidelberg, Winter.
[Eisler 1910] R. Eisler, Wörterbuch der philosophischen Begriffe, Berlin, Mittler, 3rd edition.
[Höffding 1887] H. Höffding, Psychologie im Umrissen auf Grundlage der Erfahrung, Leipzig, Fues's Verlag.
[Höfler 1897] A. Höfler, Psychologie, Wien-Prague, Tempsky.
[Külpe 1895] O. Külpe, Einleitung in die Psychologie, Leipzig, S. Hirzel; English edition [Külpe 1973].
[Külpe 1973] O. Külpe, Outline of psychology, ed. S. Sonnenschein, New York, Amo Press.
[Wundt 1909] W. Wundt, Einleitung in die Philosophie, Leipzig, W. Engelmann, 5th edition.
[Wundt 1911] W. Wundt, Grundz,ge der physiologischen Psychologie, Leipzig W. Engelmann, vol. 3, 6th edition.