The last sub-section of the previous Section has introduced us to Marc-Wogau's own account of the nature of Sense-perception. We can now explain this in greater detail by reference to what he says in Chapter V (The Relation of Sense data to Physical Objects) and Chapter VI (Sense-data and Scientific Objects).

(A) Macroscopic Objects.

     As regards ordinary macroscopic physical things, such as books, pennies, etc., Marc-Wogau's theory may be outlined as follows.

     In certain circumstances, when a person 'sees' a physical object, he visually prehends that physical object which he is said to be seeing. In other cases what he visually prehends is, not the physical object which he is said to be seeing or indeed any other physical object, but a particular which stands in a certain special relation to the visum. In the latter cases it is convenient to call the visual prehensum a 'sense-datum'; in the former it is not. If we go into greater detail, the theory may be summed up in the following six propositions.

     (1) Suppose that a certain physical object, e.g., a certain penny, is seen from various positions, under various physical and physiological conditions, and with the observer in various mental attitudes (e.g., that of ordinary practical life, that of the draughtsman, that of the introspective psychologist, etc.). Then on each occasion a different particular is visually prehended. But, provided that the positions and the conditions fall within certain limits (which can be indicated but not defined or described with complete accuracy), and that the mental attitude is that of ordinary practical life, these visual prehensa differ only abstractively and not qualitatively from each other.

     (2) Each such prehensum is identified with the seen physical abject, and IS referred to, e.g., as 'that penny there now'

    (3) (i) These prehensa form the nucleus of a larger group. The non-nuclear members are those which are prehended from positions or under conditions which fall outside the limits mentioned above. They differ qualitatively and not only abstractively from each other and from the members of the nuclear group. (ii) Each of these too is commonly spoken of as if it were a physical Abject, and as if it were the same physical object as that which is identified with each member of the nuclear group. It would be referred to, e.g., as 'that penny there now'. But (iii) existence in sense E₃ is ascribed only to the nuclear prehensa. The non-nuclear ones are said to be 'non-existent' or 'unreal' in that sense, though they are said to exist and to be real in senses E₁ and E₂. (If the reader will look back to the account of these various senses of 'existence', I think he will find that the last statement adds nothing substantial to what has gone before.)

     (4) Every member of a nuclear group is a physical object, and all members of any one such group are the same physical object. But no non-nuclear member is, strictly speaking, a physical object. In virtue of this difference it is proper to call the non-nuclear members 'sense-data' and to refuse to apply that name to the nuclear members.

     (5) In some cases where we see a physical object, e.g., the moon, the conditions required for prehending nuclear prehensa cannot in fact be fulfilled by any human percipient. In such cases, though we perceive the object, in the way in which supporters of the sense-datum theory think that we perceive all objects that we ever do perceive, we do not visually prehend it.

     (6) Under certain conditions we visually prehend, even in normal waking life, prehensa which are neither nuclear nor non-nuclear in the sense described above. An example would be the object which one visually prehends when one sees a reflexion of a physical object in a distorting mirror. Such visual prehensa are not, even in careless common speech, identified with any physical object. But they are related in certain specific ways to a certain physical object which is or may be seen, and perhaps also to certain other perceived or perceptible objects. (I will include them under Russell's name of 'wild' sense-data, though they are 'tame' enough in comparison with those which we prehend in dreams or waking hallucinations.) These cases, according to Marc-Wogau, do not raise any serious theoretical difficulties, because one is not inclined even prima facie to identify such a wild sense-datum with the physical object to which one assigns lt. Marc-Wogau is inclined to think that what is meant by 'assigning' a certain wild sense-datum to a certain physical object can be analysed in terms of causation.

     It remains to mention some consequences which Marc-Wogau believes to follow from his theory.

     (i) There is for him a problem about the relation between visual prehensum and actual visum only in the case where the former is non-nuclear or wild. For it is only in such cases that the Usual prehensum differs from the visum. (ii) In the case of non-nuclear visual prehensa, the relation between them and the visum IS that, although the two differ qualitatively, certain conditions are fulfilled which incline us to call the former by the name of the latter. These conditions were discussed in the account of 'existence' in senses E₂ and E₃. (iii) In accepting a causal analysis of the relation between a wild sense-datum and the physical object to which it is assigned Marc-Wogau is not faced with the well-known difficulties which a causal analysis presents to anyone who holds that no physical object is ever prehended. For he holds that physical objects are in principle prehensible and in fact very often prehended. So the physical object to which a certain wild sense-datum is assigned on causal grounds may have been prehended on other occasions.

     Before leaving this part of Marc-Wogau's theory I wish to make the following comments. It seems to me that one would not call a visual prehensum 'a penny' and would not claim to be 'seeing a penny', e.g., unless one believed or took for granted that-to put it very vaguely-there was a great deal more 'to it' than those characteristics which it is, or could be, visually prehended as having. And, if these beliefs were false, one would be mistaken in saying 'I see a penny'.

     Now Marc-Wogau, as we have seen, puts a certain interpretation on the statement 'This object, which I now visually prehend, has certain characteristics which I do not, and could not, visually prehend it as having.' The interpretation is that there are other contemporary particulars, which occupy simultaneously the same place as this visual prehensum and differ only abstractively from-it; and that these between them have many characteristics which it lacks. Now, if this interpretation be accepted, it seems to me that Marc-Wogau's theory has a certain resemblance to the theory which can be roughly described as holding that 'a physical object is a class or family of suitably inter-related sense-data'. I want to point out what I take to be the likenesses and the differences.

     The latter theory would hold

1. that each member of such a class is completely determinate in all its predicates;
2. that it is only the class or family collectively, and not each member of it severally, which can properly be described as the physical object seen; and
3. that the relation between the members is not that of differing only abstractively from each other.

(B) Microscopic and Ultra-microscopic Objects.

     Marc-Wogau considers in turn the following cases.

1. An ordinary macroscopic physical object, seen first cursorily and as a whole, and then from the same place with special attention to the parts and the details.
2. Such an object, seen first with the naked eye, and then through a microscope which reveals details invisible to the naked eye.
3. Such an object, and those minute and in principle imperceptible scientific objects of which it is said to be 'composed'.

     (1) In the first case he holds that there are two prehensa, but that there is no reason to think that they differ qualitatively. If they differ only abstractively, there is no objection to holding that both coexist in the same place, and no reason why we should not identify both with the physical object seen.

     (2) As an example of the second case he considers a line which, when viewed by the naked eye, looks continuous, but, when viewed through a microscope, appears to consist of a discontinuous linear series of dots. I think there is no doubt that the plain man would say 'The line is really discontinuous, but it looks continuous to the naked eye'.

     I am not at all sure that I understand what Marc-Wogau says on this topic. I shall therefore state in my own way what I suppose to be his account of the matter. There are really three objects to be considered. (i) The object visually prehended by the use of the naked eye. Let us call this O_e. This has existence in sense E₃. (ii) The object visually prehended by the use of the microscope. Let us call this O_m. This has existence only in senses E₁ and E₂; for it differs qualitatively from the physical object in being 'unnaturally enlarged', as we might say. tiu) A 'reduced' object, which is not visually prehended either by the use of the naked eye or by the use of the microscope. Let us call this øm. This agrees with O_e in size, but differs from it in being composed of a row of discontinuous dots. It agrees with O_m in the latter respect, but differs from it in that (a) each dot in it is smaller than the corresponding dot in O_m, and (b) the distance between any two adjacent dots in it is less than the distance between the corresponding two adjacent dots in O_m. This exists in sense E₃.

     According to Marc-Wogau, as I understand him, there is no objection to holding that both O_e and O_m occupy the same place at the same time, and no reason why we should not identify each of them with the actual visum. O_e is visible, in the sense of being visually prehensible; and its parts, which are themselves visually prehensible, are continuous with each other. But O_m is not visible, in that sense; nor are any of its parts. By the naked eye it is not visible in any sense. By the microscope it and its parts are 'visible' only in the sense that something else, viz., O_m and its parts, are visually prehensible; and that these parts, though qualitatively different in certain respects from the parts of øm, correspond in certain respects to the latter.

     So the correct statement would run as follows. This line has parts of two different kinds, visually prehensible and not visually prehensible. The former are continuous with each other, and the latter are discontinuous with each other. Both are located simultaneously in the same region of space; but the former fill it continuously, and the latter occupy it discontinuously as a crowd might occupy Trafalgar Square.

     Marc-Wogau contrasts this example with the following. Suppose we have a picture composed of a large number of coloured dots very near together on a white sheet of paper. If it is viewed from a considerable distance away, it appears as a continuously coloured expanse; if it is viewed from the distance of most distinct vision, it appears as a disconiuous~ction of coloured dots. I think it is certain that the plain man would say of this picture, as he would say mutatis mutandis of the line in the previous example, 'It is really discontinuous, but it looks continuous when viewed from a considerable distance'. But Marc-Wogau holds that the two cases are fundamentally different.

     I understand his position to be as follows. The object prehended by the nearer observer, like the object prehended with the naked eye in the previous example, can be identified with the physical object seen. The object visually prehended by the distant observer, like the object prehended in the previous example by the person who uses a microscope, cannot be identified with the physical object seen. For it differs qualitatively, and not merely abstractively, from the prehensum which is identified with the actual visum. So far there is no difference between the two cases. But there is the following difference. The microscopic image is not only discontinuous; it is also unnaturally enlarged, in comparison with the object prehended by the observer who uses the naked eye. But the object prehended by the distant observer is certainly not larger, and it may be smaller, than that prehended by the nearer observer. We have therefore no reason to suppose that there exists, in sense E₃, a continuously coloured surface, which cannot be visually prehended, but which corresponds on a reduced scale to the continuously coloured surface which is prehended by the distant observer but exists only in senses E₁ and E₂.

     If I have interpreted Marc-Wogau and the plain man correctly here, the former would have to say that the latter is right in both what he asserts and what he denies in the second case; and is right in his assertion, but wrong in his denial, in the first case. The physical object seen in the second example is a discontinuous set of coloured dots, and only appears to be a continuonsly coloured expanse. But in the first example it is a continuously coloured line (as it appears to the naked eye), and it is also a discontinuously occupied line (as it appears when viewed through the microscope). Nevertheless, the microscopic image and its separate items cannot be identified with the physical object and its discontinuous parts.

     (3) Marc-Wogau holds that the solution which he has given in the case of features revealed only by the microscope can be applied to features which are not and could not be revealed to sight by any optical instrument but are onb postulated to account for certain observable phenomena. There is no contradiction between ascribing continuity to the visible surface of a macroscopic physical object and discontinuity to the nltramicroscopic scientific objects which are held to occupy the region of space which it encloses. There is therefore no excuse for talking, as Eddington sometimes did, of the coloured continuous visible table as 'unreal', and the colourless discontinuous set of invisible scientific objects which occupy the same place as alone 'real'. Both may be 'real' in precisely the same sense.

     One point remains to be noticed. Marc-Wogau considers that the solution proposed above does not commit him to any particular analysis of propositions about features which are revealed only by the microscope; or about features which could not be perceived by any means, but are postulated only in order to explain certain perceptible facts. He mentions, without criticising here, three alternative analyses.

1. If it be held that we can have a clear positive conception of objects which are in principle incapable of being perceived, there is no difficulty in interpreting such propositions quite literally.
2. Failing this, we might resort to a special kind of analogy. We might describe any such feature as standing to the smallest object that could be perceived by the naked eye in the same relation of magnitude as a certain one perceptible object, e.g., a flea, stands to a certain other, e.g., the dome of St. Paul's Cathedral. This device is often adopted by popular lecturers on astronomy or on atomic physics.
3. Lastly, one might try to interpret all such propositions as conditional, e.g., as making assertions about what would be perceived if certain conditions, themselves statable in terms of sense-perception, were to be fulfilled.