and ending it with coffee over taking it in the opposite order. The following additional works may be consulted with advantage: A. N. WHITEHEAD, Principles of Natural Knowledge, H. MINKOWSKI, Raum und Zeit. H. WEYL, Space, Time, and Matter. A. S. EDDINGTON, Report on the Relativity Theory of Gravitation. Space, Time, and Gravitation. A. A. ROBB, A Theory of Time and Space. " Absolute Relations of Time and Space. S. ALEXANDER, Space, Time, and Deity, Bk. I. B. RIEMANN, Ueber die Hypothesen welche der Geometrie zu Grunde liegen. D. M. Y. SOмmerville, Non-Euclidean Geometry. * This most important work appeared while the present book was in the press. Whitehead argues that Space-Time must be homaloidal; and he deduces the characteristic results of the General Theory of Relativity from a modification of the traditional law of gravitation, and not from supposed variations in the structure of different regions of Space-Time. CHAPTER XIII quam sedem Somnia vulgo Vana tenere ferunt, foliisque sub omnibus haerent. Et ni docta comes tenues sine corpore vitas (VIRGIL, Eneid, VI.) The Physiological Conditions of Sensations, and the Ontological Status of Sensa At the end of Chapter VIII we said that the Critical Scientific Theory of physical objects and our perception of them left two main problems on hand. One was to clear up the meanings of physical place, shape, size, date, duration, etc., and to establish their cash value in terms of those corresponding characteristics of our sensa, on which they must ultimately be founded. This task I have performed to the best of my ability in the last four chapters. The other problem was to elucidate the very obscure statement that external physical objects and our own bodies "jointly produce in us the sensa by which these external bodies appear to us." Probably any solution of this problem will be found to favour (if not actually to require) some particular view as to the nature of sensa and their ontological status in the universe. So this book will fitly end with an attempt to define the meaning and estimate the truth of the above statement. • Almost every phrase in this statement bristles with ambiguities. (1) The notion of "joint" production will be found to be far from clear, and its possible alternative meanings will have to be analysed. (2) We shall have to raise the question whether the conditions jointly produce sensations, or sensa, or both. (3) The word "production" is highly ambiguous, even when we have settled what we mean by "joint production.' It may mean a kind of creation out of nothing, or a process of ordinary causation, or a process of selection out of a mass of pre-existing material. These questions are not, of course, independent of each other. It is pretty certain that any answer that is given to one of them will cut out certain answers to the rest, and will favour certain other answers to them. But we must start by treating each question separately, and then try to view the results of our separate discussions as a whole. Without prejudice to the conclusions that we may reach when we discuss question (2), we shall find it best to start by saying that processes in external bodies and in our own jointly condition sensations, rather than that they jointly condition sensa. On our view a sensation is a complex whole, in which an objective factor (the sensum) and a subjective factor (the act of sensing) can be distinguished. Whether either of these can exist apart from the other we do not at present either assert or deny. But this at least is certain; all the sensa of whose existence I am directly aware are constituents of my sensations, and all the sensa of whose existence other observers tell me are constituents of their sensations. Hence any evidence that I may think I have that certain physical and physiological processes are necessary and sufficient to produce sensa is prima facie evidence that they are necessary and sufficient to produce sensations. It may be that they can only produce sensations by producing sensa, but this question must be left aside for the present. So, to start with, we shall talk about the production of sensations, and shall leave it an open question whether this involves the production of sensa. The Notion of Joint Production.-I think that the view of educated common-sense is that there are certain events, very definitely localised in Time and Space, which happen in my brain and are the necessary and sufficient conditions of the occurrence of each of my sensations. If I sense a practically uniform sense-object, it is thought that there is a practically uniform process in some part of my brain, which lasts as long as the sensation, and is its necessary and sufficient condition. Some, but not all, of these brain-events are supposed to be due to external physical events, such as the striking of bells, the lighting of matches, etc. Others are supposed to be due to internal causes. It is held that, even when a sensation is due to some external cause, such as the striking of a bell, this is never a sufficient condition. Something must be transmitted from the external object to the sense-organ, and something must be transmitted from the sense-organ to the brain. Otherwise the brain-event, which is supposed to be the necessary and sufficient condition of the occurrence of the sensation, will not happen, and so the sensation will not be produced. I propose first to introduce some necessary technical terms for stating the common-sense view; then to clear up certain ambiguities in the notion of necessary and sufficient conditions; and then to ask in what sense, if any, there is reason to believe that certain definitely localised brain-events are the necessary and sufficient conditions of each of my sensations. (a) Originative, Transmissive and Productive Conditions. -On the ordinary view, the production of a sensation by an external physical event requires the fulfilment of at least three types of condition. Let us take the case of hearing a certain stroke of a certain bell. (1) The bell must be struck, or I shall not hear any sound characteristic of it at the time. This may be called the originative condition. (2) Unless there be air or some other material medium between my body and the bell I shall hear nothing, even though the bell be struck. There are excellent reasons, some of which have been mentioned in Chapter X, for holding that something travels with a finite velocity from where the bell is, through the medium, to my body. This may be called an external transmissive condition for my sensation of sound. (3) We have reason to think that, even though the originative and the external transmissive conditions for the occurrence of a sensation be fulfilled, no sensation will happen unless a certain nerve be intact, leading from the sense-organ to the brain. And it is generally held that the process in the nerve is transmissive in character. The evidence for this is fairly good. (a) If the nerve be cut at any point, no sensation of the kind will henceforth be experienced. Its integrity is therefore a necessary condition. (B) It is possible to note the time when an external stimulus acts on a senseorgan, and to get the patient to press a button as soon as he can after getting the sensation. If this button stops a clock, and the clock be delicate enough, there will always be a lapse of time between the two events. This, of course, does not conclusively prove that there is any lapse of time between the reception of the stimulus and the occurrence of the sensation, since the observed lapse might simply be the time between having the sensation and pressing the button. We have direct experimental evidence that a process, which takes time, travels along motor-nerves to muscles. So far as I am aware, we have no direct experimental evidence that a process which takes time travels up a sensory nerve from the stimulated organ to the brain. Still, it is reasonable to suppose that this is so, and it is in fact always assumed. On this assumption, we may say that there is an internal transmissive condition which is |