In order to answer this you must first understand the idea of 'perception'. Perception refers to the interpretation of what we take in through our senses, in terms of optical illusions this means our eyes. Optical illusions occur because our brains our trying to interpret what we see and make sense of the world around us. Optical illusions simply trick our brains into seeing things which may or may not be real.
In order to make sense of the world, our brains try to see patterns or shapes that we can easily recognise. This principle is called 'grouping'. Our brains can group things into four types:
Many cognitive psychologists hold that, as we move about in the world, we create a model of how the world works. That is, we sense the objective world, but our sensations map to percepts, and these percepts are provisional, in the same sense that scientific hypotheses are provisional (cf.
in the scientific method). As we acquire new information, our percepts shift. Abraham Pais' biography refers to the 'esemplastic' nature of imagination. In the case of visual perception, some people can actually see the percept shift in their mind's eye. Others who are not picture thinkers, may not necessarily perceive the 'shape-shifting' as their world changes. The 'esemplastic' nature has been shown by experiment: an ambiguous image has multiple interpretations on the perceptual level.Just as one object can give rise to multiple percepts, so an object may fail to give rise to any percept at all: if the percept has no grounding in a person's experience, the person may literally not perceive it.
Unlike the structuralism view of perception that used a bottoms up approach by adding up the parts, Gestalt stressed the...