The article explores the anatomical organization and adaptive functions of the brain's default network, a set of interacting hubs and subsystems that play a crucial role in internal mentation. The default network, characterized by increased activity during passive states, is composed of regions such as the medial prefrontal cortex (MPFC), posterior cingulate cortex (PCC), and medial parietal cortex. Recent studies using structural and functional imaging techniques, as well as behavioral research on mind wandering, suggest that the default network is involved in various cognitive processes, including memory, social cognition, and emotion. The network is divided into distinct subsystems, such as the MTL subsystem and the dMPFC subsystem, each contributing differently to internal mentation. The MTL subsystem is involved in memory-based construction and simulation, while the dMPFC subsystem plays a role in introspection about mental states. The article also discusses the adaptive role of the default network in spontaneous internal mentation, highlighting its potential benefits in simulating future scenarios and preparing for the future. Despite the network's widespread connectivity, its precise anatomical organization and functions remain areas of ongoing research.The article explores the anatomical organization and adaptive functions of the brain's default network, a set of interacting hubs and subsystems that play a crucial role in internal mentation. The default network, characterized by increased activity during passive states, is composed of regions such as the medial prefrontal cortex (MPFC), posterior cingulate cortex (PCC), and medial parietal cortex. Recent studies using structural and functional imaging techniques, as well as behavioral research on mind wandering, suggest that the default network is involved in various cognitive processes, including memory, social cognition, and emotion. The network is divided into distinct subsystems, such as the MTL subsystem and the dMPFC subsystem, each contributing differently to internal mentation. The MTL subsystem is involved in memory-based construction and simulation, while the dMPFC subsystem plays a role in introspection about mental states. The article also discusses the adaptive role of the default network in spontaneous internal mentation, highlighting its potential benefits in simulating future scenarios and preparing for the future. Despite the network's widespread connectivity, its precise anatomical organization and functions remain areas of ongoing research.