Such details are not suitable for exposition in these lectures, and I have dealt with them fully elsewhere[5].
[5] Cf.

_An Enquiry concerning the Principles of Natural Knowledge_, Cambridge University Press, 1919.
It is more convenient for technical purposes to look on a moment as being the class of all abstractive sets of durations with the same convergence.

With this definition (provided that we can successfully explain what we mean by the 'same convergence' apart from a detailed knowledge of the set of natural properties arrived at by approximation) a moment is merely a class of sets of durations whose relations of extension in respect to each other have certain definite peculiarities.
We may term these connexions of the component durations the 'extrinsic' properties of a moment; the 'intrinsic' properties of the moment are the properties of nature arrived at as a limit as we proceed along any one of its abstractive sets.

These are the properties of nature 'at that moment,' or 'at that instant.' The durations which enter into the composition of a moment all belong to one family.

Thus there is one family of moments corresponding to one family of durations.