Psychology has roots in both philosophy and biology. This dual relation with philosophy and biology is probably most evident with sensation and perception. Philosophers have been interested in sensation and perception since our senses put us in contact with the external world; our perceptions are our representations of the external world. René Descartes believed that perception was among our innate abilities. John Locke and other empiricists, however, argued against innate abilities, instead suggesting that knowledge is based on experience obtained through the senses or by reflection on residual sensory stimulation. Locke separated our experiences into primary and secondary qualities. Primary qualities are inherent to an object (e.g., size) whereas secondary qualities are added to an object (e.g., color). George Berkeley disagreed with Locke's notion ...