Most of what men "know" is not necessarily personally attained knowledge.
It is built on the testimony of others, on authorities, books, and the like.
As in a court of law when a person has to give testimony and is asked to tell only what he, himself, has witnessed, it often turns out that he really knows very little, that most of his knowledge is supposition or hearsay or just guesswork based on fragmentary information.
Thus, when speaking of real knowledge, it is important to recognize that it is a gradually growing thing, a combination of direct personal experience and of that which comes from critical reading and study as well as hearsay and the word of authority.
A witness is one upon whose testimony we can rely.
He has knowledge that combines the certainty of direct experience with substantiated learning.
As an example of such witnessing, let us take a simple phenomenon in nature familiar to all, the ordinary iron magnet.
We may be totally unable to grasp the meaning of it, and to hear about it second hand–of one thing pulling another by some mysterious unseen cord or psychic influence–may lead to odd distortion of images.
But once the little magnet is seen for what it is, and we observe the way it attracts or repels, the mystery is resolved.
We may not understand more, but we can bear witness that such a thing exists.
So, too, is it maintained that Israel bears witness.
The Jews can enter into Jerusalem below and know its many-sided beauty.
And although no one can go to the Jerusalem above and report back on its glory, the tribes of Israel are said to be reliable witnesses, for they have knowledge from their direct experience of the lower Jerusalem.
–Rabbi Adin Steinsaltz
From "Sanctity and Restraint" in The Candle of God by Rabbi Adin Steinsaltz