Logic is the most basic ism. It can be defined as just this, the ism that all other isms are dependent upon. As such it only describes the very basic rules that allow for human communication and thought - it gives us the tools to determine whether a statement (a loga) is true or false.
When using shells of truth, logic is the first shell.
The statement logic must be preserved refers to the fact that if logic is false, then every other ism is false, and existence itself becomes undefined and "broken".