Professor Konrad Marc-Wogau's Die Theorie der Sinnesdata (Uppsala: A.-B. Lundoquistska Bokhandeln, pp. vi, 449) is an exceptionally comprehensive book. Four closely connected strands can be distinguished in it.

1. A very full, fair, and clear synopsis of nearly all the most important books and papers by writers who have either used, or considered and explicitly rejected, the 'sense-datum' terminology in discussing the philosophy os sense perception.
2. A positive theory on this subject, which differs from that held by any of these philosophers.
3. An elaborate discussion of certam general philosophical concepts, such as the notions of Thing, Existence, Individual, etc. Marc-Wogau holds that such a discussion is needed, both as the basis for his own theory, and because the writers with whom he is concerned have used these notions uncritically and without drawing the necessary distinctions.
4. Criticisms, both in principle and in detail, on the arguments used and the conclusions reached by the authors whose theories are summarized. These criticisms are based partly on the general results reached in what I have called Strand 3, and are partly independent of these. Their primary object is to show that conclusions at variance with Marc-Wogau's own theory are at any rate unproven. But many of them are of independent interest.

(A) Preliminary Remarks on Terminology.

       Before going further I wish to state and exemplify the terminology which I am going to use in discussing the problems raised by Marc-Wogau. It is obviously very important to have a terminology which does not beg any of the numerous questions at issue in his book. It may be remarked that Marc-Wogau confines himself almost entirely to cases of visual perception. I shall do this also, though I think that the tendency among philosophers to make this restriction has had a most unfortunate effect on their treatment of the problems of sense-perception. In any case the terminology which I shall use could very easily be adapted to tactual and to auditory perception.

      The most general term which I shall use is Visual Experience. A feature common and peculiar to all such experiences is that they are experiences of colour, if we include under that heading black, white, and grey as well as the various hues.

      Now visual experiences may be sub-divided as follows. There are some which the person who is having them would naturally describe at the time by saying 'I am seeing so-and-so' or 'I seem to see so-and-so', or which he would describe in retrospect by saying 'I saw so-and-so' or 'I seemed to see so-and-so'. We will call these Ostensible Visual Perceptions. There are others which the experient would naturally describe by saying 'I had such and such a mental image' or 'I saw so-and-so in my mind's eye'. We will call these Visual Imagings. We shall be concerned only with Ostensible Visual Perceptions.

      An ostensible visual perception can be considered from two points of view.

1. We can ask: 'Is there in fact an object answering to the description which the experient gives of the object which he claims to be seeing, and is he in fact perceiving such an object?' This question can be put in the form: 'Is the ostensible visual perception veridical or non-veridical?'
2. We can ask: 'Is the ostensible visual perception evoked by light coming to the percipient's eyes from an external object?' This question can be put in the form: 'Is the ostensible visual perception normally evoked or abnormally evoked?'

      I think that the word 'see' is commonly used in such a way that it would be incorrect to say that a person was 'seeing' so-and-so unless one believed that he was having an ostensible visual perception which was both veridical and normally evoked. Suppose that one was describing what one knew to be a dream or a waking hallucination. Then, even if one knew that it corresponded to some distant contemporary scene to a degree beyond all question of chance-coincidence, one would say 'He (or I) seemed to see so-and-so', and not 'He (or I) saw so-and-so'. Suppose that one was describing what one knew to be a misperception. Then, even if one knew that it was normally evoked, one would use some such phrase as, e.g., 'I seemed to see a man pointing a gun at me, but what I really saw was a scarecrow in the twilight'.

      Now, whatever may be the case with certain other kinds of sensible experience, e.g., those of taste or of smell, the following seems to be true of all or nearly all visual experiences, and certainly of all those which could be described as ostensible visual perceptions. It is natural to describe such experiences by using a transitive verb, viz., 'see' or 'seem to see', followed by a grammatical object which consists of a general name or an explicit description, e.g., 'a penny', 'a flash of lightning', 'a brown fiat disk', 'a blue cross on a red field', and so on. I shall express the fact which I take to lie behind this linguistic usage by saying that in any ostensible visual perception there can be distinguished two factors, viz., an act of awareness and an object of awareness.

      Now suppose that a person is describing an ostensible visual perception which he is having. He will very often use the expression 'I see a so-and-so', where 'so-and-so' is either a general name for a class of material things, e.g., 'penny', or is an explicit description of a material thing, e.g., 'flat round disc made of copper and used as a coin'. If he does this, he is using a description which plainly asserts or implies a great deal more than is guaranteed by his present visual experience alone. E.g., to call an object a 'penny' implies that it is something that could be felt as well as seen, something which other people could see and touch, something made of copper and issued by the British mint as a coin, and so on. The expression 'I see a penny' might be used equally by a person in any of the following situations.

1. A person who was looking straight down on a penny lying on his table before him.
2. A person who was looking so obliquely at a penny lying on his table that it looked elliptical and not round.
3. A person under hypnotic suggestion who had a visual hallucination of seeing a penny on his table, though there was in fact no such object there at the time.

      For these reasons it is essential to distinguish conceptually the following two notions, viz.,

1. thc object which is visually prehended, and
2. the object which is ostensibly seen, in any ostensible visual perception.

      The examples given above illustrate the following facts.

1. There may be nothing at all corresponding to the ostensible visum of an ostensible visual perception, i.e., nothing actual which remoteiy answers to the description of that possible object. In that case the visual experience may be described as wholly delusive. But there will still be a certain visual prehensum, and there may be no reason whatever to doubt that it has the properties which the experient would ascribe to it.
2. The description of the ostensible visum will always include or imply properties which are not visually prehended in the prehensum. Some of these may be properties which could not be visually prehended, e.g., coldness and smoothness. Some of them may be properties which could not be sensibly presented at all, because they involve what I call 'categorial characteristics' such as cause and substance. Examples would be causal and dispositional properties, such as impenetrability, mass, etc.
3. The description of the ostensible visum may include or imply a certain determinate characteristic which could be visually prehended but which differs specifically from one which the prehensum is prehended as having. E.g., when the penny is viewed very obliquely the prehensum is prehended as elliptical, but the description of the ostensible visum as a "penny" implies that the latter is round. But in this case there is something answering to the description of the ostensible visum. The ostensible visual perception here may be said to be wholly veridical. For the ellipticity which the prehensum is prehended as having does not lead the percipient to ascribe ellipticity to the visum; on the contrary, in conjunction with certain other features in the total prehensum, it leads him to ascribe roundness to the visum.
4. There are plenty of intermediate cases where there is good reason to believe that an ostensible visual perception is neither wholly delusive nor wholly veridical. There is something actual which answers in the main to the description of the ostensible visum, and which the percipient is perceiving; but it differs in certain details from the latter. In such cases we may say that the visual experience is veridical in principle but delusive in certain details.

      When an ostensible visual perception is not wholly delusive we can talk of its actual visum, and we can compare this with the description of that possible object which is its ostensible visum.

      I can hardly hope that this terminology is impeccable, but I hope that it may serve. It is intended to leave open for discussion the following questions.

1. Suppose that there is an actual visum, answering completely or in principle to the description of the ostensible visum of a certain visual experience. What is the relation of the visual prehensum to the actual visum? Are they necessarily different entities, inter-related in some specially intimate way? Or is it possible that the visual prehensum and the actual visum may be one and the same entity under different descriptions, as, e.g., the Prime Minister of England and the Leader of the House of Commons might be?
2. If this is possible, is there any good reason for or against holding that it is true?
3. Is it possible for a visual prehensum to have characteristics besides those which it is visually prehended as having, e.g., to be in fact cold and smooth as well as brown and elliptical?
4. Is it possible for a visual prehensum to have characteristics incompatible with some of those which it is visually prehended as having, e.g., to be in fact round though it is visually prehended as elliptical?

It is plain that the answers to Questions 3 and 4 may have an important bearing on the answers to Questions 1 and 2. These are the kind of questions which Marc-Wogau is mainly concerned to discuss, and I am naturally anxious to use a terminology which shall not beg any of them.

(B) Preliminary Sketch of Marc-Wogau's Theory.

      We can now give a very rough preliminary sketch of Marc-Wogau's main contentions in Strands 2 and 3. I think that the following are the most important points.

      (1) He thinks that most writers who would commonly be described as supporters of some form of the Sense-datum Theory have held that the visual prehensum and the actual visum are different entities even when the visual perception is wholly veridical. They have held various views about the nature of the actual visum, and correspondingly various views about the nature of the relation between the visual prehensum and the actual visum. But they have never admitted that the visual prehension and the actual visum could be one and the same entity under different descriptions; and they have produced many arguments to show either that this is impossible or that it is never in fact true.

      (2) Most of them have either explicitly asserted or tacitly assumed that a visual prehensum can have no characteristics besides those which it is visually prehended as having, and a fortiori that it can have no determinate characteristic incompatible with any which it is prehended as having. If either of these contentions is admitted, Question 1 above must be answered in the following way. A penny, e.g., is certainly a round hard thing made of copper; and what I visuallv prehend when I look at a penny is never prehended as hard or made of copper, whilst it is prehended as elliptical and not as round if I am looking very obliquely at the penny. Therefore, unless what I visually prehend can have certain characteristics besides those which I prehend it as having, and certain determinate characteristics incompatible with some of those which I prehend it as having, it cannot be identical with the penny which is the actual visum.

      (3) Marc-Wogau holds that in certain cases, e.g., when a penny is looked at directly and not very obliquely from about arm's length in normal light by a person in the mental attitude of ordinary practical life, there is no reason to doubt that what is visually prehended is in a certain sense identical with the actual visum, i.e., with the penny itself. In other cases, e.g., when the penny is looked at in a very oblique direction or from a very great distance or when the observer is in certain other mental attitudes, the visual prehension is certainly not identical in that sense with the actual visum. It is then an entity which stands in a certain peculiar relation to a certain other qualitatively different entity, viz., the penny itself. In such cases, and a fortiori in wilder cases, such as vision in a mirror, waking hallucination, dreaming, etc., it is proper to speak of the visual prehensum as a 'sense-datum'.

     It will now be clear why Marc-Wogau discusses certain very general problems which are not usually treated explicitly by writers on the philosophy of sense-perception. The fundamental question at issue is whether the prehensum and the actual visum of a visual perception are ever identical or must always be different. Therefore the various meanings of 'identity' anti 'difference' need to be discussed. Again, the actual visum is in general a material thing, e.g., a penny. Therefore it is necessary to discuss the notion of 'material thing'. Therefore it is desirable to consider the following two notions, viz.,

1. the identity of a thing through time and change, and
2. the relations between one thing and its many appearances at a given moment.

     Now among the reasons which have been given for holding that the visual prehensum and the actual visum are always different the following have been prominent.

1. It has been alleged that the prehensum of any visual experience certainly exists, whilst it is always possible to doubt whether anything exists answering to the description of the ostensible visum.
2. It has been alleged that the visual prehensum is always a particular individual, whilst a material thing is of a different logical type, e.g., a class.

       Marc-Wogau's discussions of these several very general problems occur at various parts of his book, interspersed between the treatments of much more detailed problems specially concerned with sense-perception. It seems to me to be convenient, now that I have explained why Marc-Wogau thinks it necessary to deal with these general problems, to take them all together and to clear them out of the way at the beginning. For Marc-Wogau's arguments and conclusions about specific problems in the philosophy of sense-perception depend in part on his answers to these general questions.