It is convenient then to define a moment as the group of abstractive sets which are equal to some {sigma}-antiprime, where the condition {sigma} has this special meaning.
It will be found on consideration (i) that each abstractive set forming a moment is a {sigma}-antiprime, where {sigma} has this special meaning, and (ii) that we have excluded from membership of moments abstractive sets of durations which all have one common boundary, either the initial boundary or the final boundary.

We thus exclude special cases which are apt to confuse general reasoning.

The new definition of a moment, which supersedes our previous definition, is (by the aid of the notion of antiprimes) the more precisely drawn of the two, and the more useful.
The particular condition which '{sigma}' stood for in the definition of moments included something additional to anything which can be derived from the bare notion of extension.

A duration exhibits for thought a totality.

The notion of totality is something beyond that of extension, though the two are interwoven in the notion of a duration.
In the same way the particular condition '{sigma}' required for the definition of an event-particle must be looked for beyond the mere notion of extension.