See? you used the term "spiritual principle", which is correct and acceptable. However, it would be wrong to say that Gandhi used "spiritual science" in his struggle.

One can say that
principles are a set of rules/instructions that one adheres to (and sometimes, blindly follow). Science, and specifically, the scientific method, on the other hand is not only a set of rules or instructions--it's a tool...a way, a method, a process of thinking and doing things, and in doing so, it aspires to eliminate conditions that would tend to "muddle" logic and reason.
-RODION