There are within it antecedents and consequents which are also durations which may be the complete specious presents of quicker consciousnesses.

In other words a duration retains temporal thickness.

Any concept of all nature as immediately known is always a concept of some duration though it may be enlarged in its temporal thickness beyond the possible specious present of any being known to us as existing within nature.

Thus simultaneity is an ultimate factor in nature, immediate for sense-awareness.
Instantaneousness is a complex logical concept of a procedure in thought by which constructed logical entities are produced for the sake of the simple expression in thought of properties of nature.

Instantaneousness is the concept of all nature at an instant, where an instant is conceived as deprived of all temporal extension.