198 HUME ix 



So in mineralogy, a crystal of a definite chemical 

 composition may have its substance replaced, 

 particle by particle, by another chemical com- 

 pound. When does it lose its primitive identity 

 and become a new thing? 



Again, a plant or an animal, in the course of its 

 existence, from the condition of an egg or seed to 

 the end of life, remains the same neither in form, 

 nor in structure, nor in the matter of which it is 

 composed: every attribute it possesses is con- 

 stantly changing, and yet we say that it is always 

 one and the same individual. And if, in this case, 

 we attribute identity without supposing an in- 

 divisible immaterial something to underlie and 

 condition that identity, why should we need the 

 supposition in the case of that succession of 

 changeful phenomena we call the mind? 



In fact, we ascribe identity to an individual 

 plant or animal, simply because there has been no 

 moment of time at which we could observe any 

 division of it into parts separated by time or space. 

 Every experience we have of it is as one thing 

 and not as two; and we sum up our experiences 

 in the ascription of identity, although we know 

 quite well that, strictly speaking, it has not been 

 the same for any two moments. 



So with the mind. Our perceptions flow in 

 even succession; the impressions of the present 

 moment are inextricably mixed up with the 

 memories of yesterday and the expectations of 



