Systems are collections of related things. These things could be mechanical controls, computers, ideas, processes, or anything else. Systems science studies these things and their interrelation. Given this broad definition, systems science is naturally cross-disciplinary; and thus its methods are typically applicable to all other traditional disciplines.
The systems thinking that systems science engenders differs from traditional scientific approaches as it provides a rigorous framework for studying wholes, instead of parts. Therefore, systems thinking is not in opposition to Enlightenment reductionist thinking, but rather a complement to it.