The human brain can be thought of as having two different modes. Daniel Kahneman, an influential psychologist, labels one the "fast" mode of thinking and the other the "slow" mode. In the fast mode, according to Kahneman, we make decisions using our intuitions and emotions. The sight of a spider may activate the fast mode and lead a person to scream. The same person's slow mode might decide (after a moment) that the spider is not actually a threat. In the slow mode, we make decisions using reason and logic. The slow mode is helpful in complex situations where the right choice is not always clear. However, because the slow mode uses more mental resources than the fast mode, our brains can't rely on it all the time. The instincts that drive the fast mode of thinking save us a great deal of time and energy.