Dual process theory in simple terms
Dual process theory, made famous by Daniel Kahneman's Thinking, Fast and Slow, says the mind has two ways of thinking. System 1 thinking is fast, automatic and intuitive: it gives you an answer before you know you asked, like recognising a friend's face or leaping away from a charging dog. System 2 thinking is slow, deliberate and effortful: it works through a problem step by step, like doing mental arithmetic or wondering why a dog is chasing its own tail.
What neuroscience adds
The two systems are ideas from cognitive psychology, not places in the brain. Brain imaging finds no separate System 1 region and System 2 region. Instead, fast thinking leans on specialised perceptual, memory and emotional circuits (visual cortex, hippocampus, amygdala), while slow thinking strongly and persistently recruits the frontoparietal control network (dorsolateral prefrontal cortex, intraparietal sulcus, anterior cingulate, anterior insula), quietens the default mode network, and increases coordination between distant regions. This map shows those differences in intensity, timing and connectivity on one shared brain.