Die innere Notwendigkeit,
The inner necessity
dass das Wissen Wissenschaft sei, liegt in seiner Natur, (…).
that knowledge should be science lies in its very nature;
die äußere Notwendigkeit,
the outer necessity
aber (…) ist dasselbe, was die innere, in der Gestalt nämlich, wie die Zeit das Dasein ihrer Momente vorstellt.
however, (…) is the same as the internal: it lies in the shape in which the process of time presents the existence of its moments.
"internal", that is a necessity proven within the text, is the same as "external" necessity, history, that is the necessity of the Preface, where historical circumstances can be discussed. Briefly: A Preface to a philosophical work is impossible, but, coming from outside, it reaches within the text, touches it.