It can be stated in this way: There are durations which contain as parts any two durations of the same family.

For example a week contains as parts any two of its days.

It is evident that a containing duration satisfies the conditions for belonging to the same family as the two contained durations.
We are now prepared to proceed to the definition of a moment of time.
Consider a set of durations all taken from the same family.

Let it have the following properties: (i) of any two members of the set one contains the other as a part, and (ii) there is no duration which is a common part of every member of the set.
Now the relation of whole and part is asymmetrical; and by this I mean that if A is part of B, then B is not part of A.Also we have already noted that the relation is transitive.

Accordingly we can easily see that the durations of any set with the properties just enumerated must be arranged in a one-dimensional serial order in which as we descend the series we progressively reach durations of smaller and smaller temporal extension.