152 MYSTICISM AND LOGIC 



Logically a sense-datum is an object, a particular ol 

 which the subject is aware. It does not contain the 

 subject as a part, as for example beliefs and volitions do. 

 The existence of the sense-datum is therefore not logically 

 dependent upon that of the subject ; for the only way, 

 so far as I know, in which the existence of A can be 

 logically dependent upon the existence of B is when B 

 is part of A . There is therefore no a priori reason why a 

 particular which is a sense-datum should not persist 

 after it has ceased to be a datum, nor why other similar 

 particulars should not exist without ever being data. 

 The view that sense-data are mental is derived, no doubt, 

 in part from their physiological subjectivity, but in part 

 also from a failure to distinguish between sense-data and 

 " sensations." By a sensation I mean the fact consisting 

 in the subject's awareness of the sense-datum. Thus a 

 sensation is a complex of which the subject is a con- 

 stituent and which therefore is mental. The sense-datum, 

 on the other hand, stands over against the subject as that 

 external object of which in sensation the subject is 

 aware. It is true that the sense-datum is in many cases 

 in the subject's body, but the subject's body is as dis- 

 tinct from the subject as tables and chairs are, and is in 

 fact merely a part of the material world. So soon, there- 

 fore, as sense-data are clearly distinguished from sensa- 

 tions, and as their subjectivity is recognised to be physio- 

 logical not psychical, the chief obstacles in the way of 

 regarding them as physical are removed. 



V. ' SENSIBILIA ' AND ' THINGS ' 



But if " sensibilia " are to be recognised as the ultimate 

 constituents of the physical world, a long and difficult 

 journey is to be performed before we can arrive either at 



