The future is the indefinite time period after the present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the apparent nature of reality and the unavoidability of the future, everything that currently exists and will exist can be categorized as either permanent, meaning that it will exist for the whole of the future, or temporary, meaning that it won't and thus will come to an end. The future and the concept of eternity have been major subjects of philosophy, religion, and science, and defining them non-controversially has consistently eluded the greatest of minds. In special relativity, the future is considered absolute future, or the future light cone