The idea that there are different kinds of memory is old. Aristotle, for instance, famously distinguished between memory and reminiscence, the former roughly corresponding to the retention of temporally based information from past events, and the latter to the act of retrieving previously stored information. Cognitive psychologists and neuroscientists also agree that remembering is a diachronic process, but they tend to think that it involves three stages: encoding, storage, and retrieval. Declarative or explicit memory is often divided into two subclasses as well, thanks to Tulving's influential distinction between episodic memory and semantic memory. The shifting of memories from one system to another is thought to aid in overall memory storage by preventing the hippocampus from becoming too overloaded with memories. The precise timescale for systems consolidation is a matter of some debate; it is thought to occur anywhere from a few weeks or months to possibly even years or decades after the initial encoding.