XII. ILLUSIONS, HALLUCINATIONS, AND DREAMS

It remains to ask how, in our system, we are to find a place for sense-data which apparently fail to have the usual connection with the world of physics. Such sense- data are of various kinds, requiring somewhat different treatment. But all are of the sort that would be called " unreal/' and therefore, before embarking upon the dis- cussion, certain logical remarks must be made upon the conceptions of reality and unreality.
Mr. A. Wolf1 says :
" The conception of mind as a system of transparent activities is, I think, also untenable because of its failure to account for the very possibility of dreams and hallu- cinations. It seems impossible to realise how a bare, transparent activity can be directed to what is not there, to apprehend what is not given."
This statement is one which, probably, most people would endorse. But it is open to two objections. First it is difficult to see how an activity, however un- " trans- parent," can be directed towards a nothing : a term of a relation cannot be a mere nonentity. Secondly, no reason
t " Natural Realism and Present Tendencies in Philosophy," Proc, Arist. Sac., 1908-1909, p. 165.
174 MYSTICISM AND LOGIC
is given, and I am convinced that none can be given, for the assertion that dream-objects are not " there ' and not " given." Let us take the second point first
(1) The belief that dream-objects are not given comes, I think, from failure to distinguish, as regards waking life, between the sense-datum and the corresponding
' thing." In dreams, there is no such corresponding " thing ' as the dreamer supposes ; if, therefore, the
' thing ' were given in waking life, as e.g. Meinong maintains,1 then there would be a difference in respect of givenness between dreams and waking life. But if, as we have maintained, what is given is never the thing, but merely one of the " sensibilia " which compose the thing, then what we apprehend in a dream is just as much given as what we apprehend in waking life.
Exactly the same argument applies as to the dream- objects being " there." They have their position in the private space of the perspective of the dreamer ; where they fail is in their correlation with other private spaces and therefore with perspective space. But in the only sense in which " there " can be a datum, they are " there ' just as truly as any of the sense-data of waking life.
(2) The conception of " illusion " or " unreality," and the correlative conception of " reality," are generally used in a way which embodies profound logical con- fusions. Words that go in pairs, such as " real ' and "unreal," "existent" and "non-existent," "valid" and " invalid," etc., are all derived from the one funda- mental pair, " true " and " false." Now " true " and " false " are applicable only — except in derivative signifi- cations— to propositions. Thus wherever the above pairs can be significantly applied, we must be dealing either with propositions or with such incomplete phrases as
1 Die F.rfahrungsgrundlagen unseres Wissens, p. 28.
SENSE-DAiA AND PHYSICS 175
only acquire meaning when put into a context which, with them, forms a proposition. Thus such pairs of words can be applied to descriptions,1 but not to proper names : in other words, they have no application whatever to data, but only to entities or non-entities described in terms of data.
Let us illustrate by the terms " existence " and " non- existence." Given any datum x, it is meaningless either to assert or to deny that x " exists/' We might be tempted to say : " Of course x exists, for otherwise it could not be a datum." But such a statement is really meaningless, although it is significant and true to say ' My present sense-datum exists," and it may also be true that " x is my present sense-datum." The inference from these two propositions to ' x exists ' ' is one which seems irresistible to people unaccustomed to logic ; yet the apparent proposition inferred is not merely false, but strictly meaningless. To say " My present sense-datum exists " is to say (roughly) : " There is an object of which ' my present sense-datum ' is a description." But we cannot say : : There is an object of which ' x ' is a description," because ' x ' is (in the case we are supposing) a name, not a description. Dr. Whitehead and I have explained this point fully elsewhere (loc. cit.) with the help of symbols, without which it is hard to understand ; I shall not therefore here repeat the demonstration of the above propositions, but shall proceed with their applica- tion to our present problem.
The fact that " existence ' is only applicable to descriptions is concealed by the use of what are gram- matically proper names in a way which really transforms them into descriptions. It is, for example, a legitimate
1 Cf. Principia Mathematica, Vol. I, * 14, and Introduction, Chap.