178 MYSTICISM AND LOGIC 



of correlation with other private worlds that makes 

 others condemn them. Omitting the latter ground, our 

 reason for condemning them is that the " things " which 

 we infer from them cannot be combined according to the 

 laws of physics with the " things " inferred from waking 

 sense-data. This might be used to condemn the " things " 

 inferred from the data of dreams. Dream-data are no 

 doubt appearances of " things," but not of such " things " 

 as the dreamer supposes. I have no wish to combat 

 psychological theories of dreams, such as those of the 

 psycho-analysts. But there certainly are cases where 

 (whatever psychological causes may contribute) the 

 presence of physical causes also is very evident. For 

 instance, a door banging may produce a dream of a naval 

 engagement, with images of battleships and sea and smoke. 

 The whole dream will be an appearance of the door bang- 

 ing, but owing to the peculiar condition of the body 

 (especially the brain) during sleep, this appearance is not 

 that expected to be produced by a door banging, and thus 

 the dreamer is led to entertain false beliefs. But his 

 sense-data are still physical, and are such as a completed 

 physics would include and calculate. 



(4) The last class of illusions are those which 

 cannot be discovered within one person's experience, 

 except through the discovery of discrepancies with 

 the experiences of others. Dreams might conceivably 

 belong to this class, if they were jointed sufficiently 

 neatly into waking life ; but the chief instances 

 are recurrent sensory hallucinations of the kind 

 that lead to insanity. What makes the patient, in such 

 cases, become what others call insane is the fact that, 

 within his own experience, there is nothing to show that 

 the hallucinatory sense-data do not have the usual kind 

 of connection with " sensibilia " in other perspectives. 

 Of course he may learn this through testimony, but he 



