Like perceptual illusions, cognitive biases give us a distorted understanding of the world and are caused by the way our brains naturally process complicated information. This text draws on the metaphor of illusions to explain the nature, origins, and effects of cognitive bias, highlighting two key sources of bias: cognitive heuristics and motivated reasoning. Having a better understanding of cognitive bias improves our ability to reason well by helping us to detect and correct common errors in how we seek, select, and evaluate evidence.