Subjective perception is the manner in which an individual views the physical world based on the workings of his own brain and sensory systems. Each individual has a brain, sensory systems, and cognitive structures that differ from those possessed by everyone else. As such, one cannot conclusively say that he hears or sees exactly what other people hear or see. The problem of subjective perception is very important in philosophy, brain science, and psychology, and is a significant barrier to establishing the conclusive objectivity of scientific findings. Perception can be shared only through communication, and no form of communication exists that can perfectly express one person's perception to another.
A commonly discussed example of subjective perception is the question of whether or not every person perceives color the same way. Many people, even children, come to the realization at some point that, without being in someone else's mind, they cannot know if other people see the same "yellow" as they do. The light waves that reach each individual's eyes are the same, but one cannot know if the eyes and brain process the light waves in exactly the same way. The perceptions that two different people have of the world could have subtle differences or could vary dramatically. The subjective nature of perception makes it impossible to know conclusively.
For some, subjective perception is simply an interesting issue to ponder, but it is extremely important to science. Many scientific experiments are based on observations, and non-quantitative observations are generally rooted in perception. Perception does not provide a pure representation of the physical world, though, as sensory input passes through perceptive, neurological, and cognitive filters. One cannot even know with certainty that one person's filtered view of the world is the same as that of another person. This raises the question of how science can claim to present objective findings about the world when those findings are based on subjective perception.
Throughout history, many different philosophers have pondered and written about the issue of subjective perception. Some claim that, though people cannot directly know the physical world, human perception is sufficient to allow for reasonable discussion and study of the perceived world. Others take it to an extreme and claim that there is no objective external world — or at least that the external world is completely unknowable and that attempting to explore and understand it is entirely futile.