In the section of "Republic" where Plato describes his famous allegory of the cave, he explains his ideas on perception and reality. Individuals are held to the reality that their perceptions show them, allowing others who are capable of manipulating those perceptions to define the nature of their reality. Plato explains that it is the philosopher's responsibility to free individuals from their manipulated perceptions and allow them to seek reality for themselves. He continues by explaining that you can only see the true reality for yourself without interference from others interpreting or designing it for you.
John Locke, in "An Essay Concerning Human Understanding," describes the dependency of perception on the brain. He explains that pure sense perception is useless without a cerebral interpretation. Locke uses the analogy of seeing yourself on fire to explain that you would have no reason for alarm if your brain did not interpret the sensation as painful. He explained that you see the fire, understand that it is dangerous and notice the sensation of heat as a warning sign. For Locke, perception is external, but people's interpretation of perception is internal and occurs in the brain where he said realism exists.
Empiricism follows the ideology that no sense of reality is possible without a person's ability to perceive it. It explains that sense is your primary connection to knowledge and understanding, and that without the ability to sense and perceive, you would have to rely on deduction or intuition to advance your understanding beyond what you already possess. Empiricism suggests that some understanding of unperceived reality is possible through either intuition or deduction but even these require sense practice to observe prior conditions for you to base your evaluations, and you could never consider your conclusions to represent absolute reality.
Existentialism, as it pertains to perception and reality, focuses on the nature of perceived reality both as you create it for yourself and as you experience it when created by someone else. Jean-Paul Sartre explained that the nature of social reality is, by its nature, a perceptual conflict, pitting your understanding of reality against the envisioned reality of those around you. Existentialism relies on the individual's responsibility to find meaning and reality for themselves, rather than giving in to the perceived realities around you.