Premises can mean various things; e.g.
- the theories upon which an action or decision is based,
- the assumptions upon which a logical argument is based,
- it is a legal term for a building and the land on which it stands,
- it is a legal term used when referring to an earlier statement within a document,
- It can be used in a slightly archaic way for 'the aforementioned', which is similar in meaning to 4 above.
In your case, it sounds like it has the meaning given by 5, in which case "Stoic Physics, in the premises,..." would mean something similar to:
Stoic Physics, which was mentioned earlier,...
Depending on the source of the quote, this earlier mention may have been in an introduction to the current chapter, or in a foreword to a book, or in an introductory paragraph to a monograph, etc.
"In terms whereof" is a rather archaic way of saying "in terms of which". So, this sentence means something like:
Stoic Physics, which was mentioned earlier, is a natural philosophy. Stoic Philosophers tried to understand and describe how the gods affect the universe, in terms of natural processes.