A paper I wrote for Philosophy 101
in response to the questions:
How do you know you are in touch with the real world?
How do you get from appearance to reality?
Perhaps one of the single most important issues philosophy addresses is how we as humans can know what is truly real and how we build our mental conceptual worlds from our sensory impressions of the world outside. Because we can only experience the world indirectly, it's tempting to state simply, as does the Sophic philosopher, Protagoras, that we can never know truth and that appearances are all we can ever have knowledge of.
These questions are closely related to the question, "What can be trusted?"- what can we trust if not our own senses? As I wrote in a previous paper, direct perception is not the only tool with which we as humans may acquire knowledge. Science and the scientific method have provided us other means of gaining empirical knowledge. Facts of themselves are unimportant, but patterns of information lead to knowledge and finally understanding. We can rely on mathematical logic.
When a person is born, the mind, to use Locke's phrase, is tabula rasa; there are no innate ideas, aside from possibly some genetic predispositions toward language use and a few other things, as per the work of Chomsky and others. The senses give the building blocks of all further knowledge, the evidence that something exists, and some of its attributes. What that particular thing is one learns via a process called conceptualization.
Conceptualization is our method of organizing sensory material. While forming a concept, one isolates two or more similar objects, and integrates them into a single mental unit, usually symbolized by a word. A concept subsumes a nearly unlimited number of instances: the ones originally isolated, and all others that are similar to them. Similarity is the key to conceptualization: while certain attributes of conceptual objects may change, others remain static, and thus form useful concept-patterns.
After we've formed concepts we can extend our understanding by way of mathematical methods and systems of logic that we've developed for scientific investigation which are provably consistent in and of themselves. Still, absolute certainty about reality eludes us. This, however, is largely irrelevant. What matters is that reality is objective and that we can indeed know almost anything about the world worth knowing. We may never know whether we exist in a computational matrix or in an artificially spawned sub-universe of some other world, but our own reality is knowable and existent outside of ourselves, and this is what is important.