"What then gives so great a propension to ascribe an identity to these successive perceptions, and to suppose ourselves possessed of an invariable and uninterrupted existence through the whole course of our lives? In order to answer this question, we must distinguish between personal identity as it regards our thought and imagination, and as it regards our passions, or the concern we take in ourselves. The first is our present subject; and to explain it perfectly we must take the matter pretty deep, and account for that identity which we attribute to plants and animals; there being a great analogy betwixt it and the identity of a self or person."--(I. pp. 321, 322.) Perfect identity is exhibited by an object which remains unchanged throughout a certain time; perfect diversity is seen in two or more objects which are separated by intervals of space and periods of time. But, in both these cases, there is no sharp line of demarcation between identity and diversity, and it is impossible to say when an object ceases to be one and becomes two. When a sea-anemone multiplies by division, there is a time during which it is said to be one animal partially divided; but, after a while, it becomes two animals adherent together, and the limit between these conditions is purely arbitrary. So in mineralogy, a crystal of a definite chemical composition may have its substance replaced, particle by particle, by another chemical compound. 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 constantly 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 indivisible 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 to-morrow, and all are connected by the links of cause and effect.