 Philosophers often think of reality as consisting of elements that are discovered, engineered, combined, and adapted to meet the needs of their particular systems. In thinking about those elements of reality, Western philosophy has often depended on primary concepts like things, facts, stuff, sensations, subject, object, being, substance, essence, attribute, quality, and so forth. As that list of elemental concepts has become standardized, it has served as a glossary for future thinking and further philosophical initiatives. Philosophers may discover or even craft new elements, but they do so against that preexisting background, like filling gaps within the periodic table. Commonly, those elements are thought to be fixed entities that are connected or acted upon by relational forces such as causes or agents. Change may also be understood as occurring through the interaction of forces in dynamic opposition, including dialectical opposition.