Going back, well, *almost* to the beginning:

A reconstruction of the first part of the famous Bell-LaPadula (BLP) model. Note that this is a formal mathematical model, using symbolic logic. Not the first formal model of security, nor even the first state machine model, but one of the most useful in the early days.

The second part of the famous Bell-LaPadula model is available here.

So, now you know.

Possibly easier to follow, a review of the Bell-LaPadula model, thirty years later, by David Bell.