The ways through which knowledge is acquired are three: perception, the report, and reason. We know those ways of knowledge are reliable, because when we recall the information that we know, we find that all of it has been acquired through one of those three channels.
Perception involves the acquirement of information through the senses. The senses are divided into the inward, and the outward senses.
The inward sense is introspection, by way of which you are able to acquire knowledge about yourself. For example: when you feel hungry, you know that you are hungry by introspection.
The outward senses are five: sight, hearing, smell, taste, and touch.
When the perceived object is detected with one of the five outward senses, a realization emerges in the body which the mind then interprets. The reality of perception, is this emergent realization.
For example: when light reflects off the surface of a body and enters the eye, the eye reacts and a realization emerges within it. This emergent reaction is what is called “sight”.
Sensory realizations correlate with what is perceived, since they are reactions to it. Through the mind’s interpreting of this realization, knowledge is acquired.
Notice that the outward senses, in of themselves, do not make mistakes. They are merely a window connecting the mind to the outside world. Rather, mistakes occur when one misinterprets sensory realizations.
Reports are accounts of events that we did not personally perceive, communicated to us by those who did perceive those events.
For a report to be a way to knowledge, it must be mass transmitted. Mass transmission (Tawatur) is a report’s being transmitted to us by so many independent witnesses, such that it would be nomically impossible for them to have all conspired to fabricate it.
For example: consider the existence of a foreign country that you have never been to. Despite not having personally visited this country, you still know that it exists. This is because you have received reports about this country’s existence from various sources: whether they be reports from people who lived there, or people who visited it, or even pictures of this country that you’ve seen on TV. All of those reports, when taken collectively, form mass transmitted proof for the existence of said country.
Reason is the human ability to use non-inferential knowledge, in order to acquire inferential knowledge.
For example: once the meanings of “body”, “rest”, and “motion”, are established non-inferentially for the think, he can come to know that “a body cannot exist in a state of being neither at rest nor moving”.
 “Nomically Impossible” meaning: a proposition which does not accept affirmation, because its affirmation violates normalcy.
More on nomic judgements here.