Locke's Essay concerning Human Understanding, first published in England in 1690, was a revolutionary treatise on how humans learn and is considered the foundation of modern psycology.
Disputing Aristotle's belief that there existed certain innate ideas in every culture, Locke argued that humans are born with a tabula rasa, a blank tablet, and that tablet gets filled up by actual observations as a person grows. A person's senses take in impressions of the world as that person experiences the world, and the result is that everyone has a slightly, if not profoundly, different view of the world and a different degree of knowledge. The world, according to Locke, was exactly as one's senses made it appear to be, which means that whatever one observes is one's reality. By extension of this logic, then, each person's reality is slightly different.
Because we understand reality and acquire knowledge through our own observation, Locke believed that we can never, as individuals, understand everything we encounter completely and that we have to realize that everyone, as I noted above, will understand knowledge in a slightly different way. In other words, complete knowledge of the world, because no one individual can sense enough to gain complete knowledge, was just not an attainable goal for humans.