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230 M. W. CALKINS: writes, 1 " Der Raum ist durch und durch nichts anderes als die Moglichkeit der wechselseitigen Bestimmungen seiner Theile durch einander, welche Lage heisst " ; and Spencer 2 distinguishes coexistence from succession, in that " whereas the terms of the first can be known in the reverse order with equal vividness, those of the second cannot ". Yet it is at once evident that the spatial is, to say the least, not the only form of the permanent and reversible manifold ; the notes in a scale and the terms of a numerical series are also reversible but not spatial, for even if one assert the spatial character of sounds, it is surely not by virtue of their space distinctions that the notes are capable of reversal. One is thrown back upon the question : What is the spatial, since, at best, it is only one among the forms of the reversible ? Once more, there can be no doubt of the ordinary answer : the spatial is the external, and just as time is a category of the inner, so is space a category of the outer life. But this doctrine accords ill with the common view that not all sense- qualities, but only the visual and the tactual, are spatial. Why should not sounds and odours as well as colours and surfaces have form and location? Or, if one take one's stand with the extreme nativists, like James and Ward, and affirm the spatial character of all sense- qualities, the questions still remain : What of the mathematical reversible ? is not that still independent of me and so external to me ? The true nature, like the invariable test, of externality, is its superiority to the individual, that is, its universality. The outer world is the world whose lights and sounds and fra- grance all men share, while the inner world of my imagination belongs to me alone ; the external truth is the object of common conviction, while the illusion is the product of the individual mind ; in a word, the external world is the world of society as opposed to the world of the lonely self. This impossibility of limiting the ' external ' or ' reciprocally deter- mined ' to ' the spatial,' fairly drives us at length to the conclusion which psychology has long held before us, that the spatial means something quite other than the external, and is itself nothing more than a concrete : a sense-quality or a complex of sense elements. The arguments of the Kantians against the sensuousness of the spatial are not decisive. To urge that Space is re- cognised as one, in a sense in which ' redness ' and ' softness ' are not called ' one,' is to overlook the difference between 1 Welt als Wille, u,s.w., i., p. 109. 2 Principles of Psychology, third ed., part vi., c. 22, vol. ii., p. 275.