If it is true that...

The essence of a composite substance can be signified as a whole or as a part

Then it must be true that...

The essence of a simple substance is form alone

If it is true that...

The essences of simple substances are form alone and are without matter

and

The essence of a composite substance can be signified as a whole or as a part

Then it must be true that...

The essence of a simple substance is form alone

If it is true that...

The essence of a simple substance is form alone

and

The essence or quiddity can be understood without understanding anything about its existence

Then it must be true that...

Existence is something other than the essence or quiddity, unless there is something whose quiddity is its very own existence