Flowing duration evidently implies succession, and succession involves time; for succession is a relation between something which follows and something which precedes. On the other hand, time also involves succession; whence it would seem that neither time nor succession can be defined apart from one another, the definition of the latter presupposing that of the former, and that of the former presupposing the notion of the latter. Although we need not be anxious about this point (for time and succession really involve one another, and therefore may well be included under the same definition), we must observe that the notion of succession, though ordinarily applied to duration, extends to other things also whenever they follow one another in a certain order. Thus the crust of the earth is formed by a succession of strata, the Alps by a succession of mountains, the streets of the city by a succession of houses, etc. Hence the notion of succession is more general than the notion of time, and consequently there must be some means of defining it independently of the consideration of time. Balmes explains succession, without mentioning time, in the following manner: “There are things which exclude one another from the same subject, and there are other things which do not exclude one another from the same subject. The existence of those things which exclude one another implies succession. Take a line _ABC_. A body placed in _A_ cannot pass over to the place _B_ without ceasing to be in _A_, because the situation _B_ excludes the situation _A_, and in a similar manner the situation _C_ excludes the situation _B_. If, then, notwithstanding this mutual exclusion, the three places are really occupied by the same body, there is succession. This shows that succession is really nothing else than _the existence of such things as exclude one another_. Hence succession implies the existence of the thing that excludes, and the non-existence of the things that are excluded. All variations involve some such exclusion; hence all variations involve succession.… To perceive the existence of things which exclude one another is to perceive succession and time; to measure it is to measure time.” Thus far Balmes.[10]