Intelligence can be considered a process, a product, and an institution. Institutions in particular point toward the idea of national security, since intelligence services are curiously bound up with both state sovereignty and the core executive. Preemption is perhaps the most important idea that has served to enhance the importance of intelligence. One of the most enduring definitions of intelligence is that it is a special form of information that allows policy makers, or operational commanders, to make more effective decisions. Quite often this intelligence is secret in nature, consisting of information that an opponent does not wish to surrender and actively seeks to hide. And although it is widely accepted that intelligence studies as a field is under-theorized, some areas have received more attention than others. Perhaps because policy makers have seen warning against surprise attack as one of the highest priority intelligence requirements, this area has been the most fully conceptualized. In addition, intelligence agencies themselves have frequently advanced the claim that their ability to lend a general transparency to the international system improves stability. Also, these agencies not only gather intelligence on world affairs but also seek to intervene covertly to change the course of events. Another controversial aspect of intelligence involves the cooperation between intelligence and security services.