Almost all knowledge is quite naturally a combination of that which is sensorially perceived by oneself and of credible information received from others.
Just as on hearing a report from a traveler returning from an unknown country, one allows oneself to believe in the plausible and to entertain doubt about the rest.
The difference lies in the degree of clarity and coherence of the information, which depends also on the scope of one's previous knowledge.
But, ultimately, there always remains a certain amount of the unknown, the "mysterious."
And it is this that requires penetration and inquiry until it becomes acceptable, credible, so to speak, and is resolved into the certainty of which it is written, "And you shall know this day…. "
But the witness cannot be dispensed with; we have to receive the testimony of someone who has been there.
The mitzvot thus serve as witnesses.
They come from the higher worlds.
They are projections of higher worlds upon this world of ours.
Yet still they remain no more than witnesses or rather "something else" that has penetrated to our being here in this world.
Thus, the Torah itself may be seen as witness.
Indeed, it has been called the testimony or Testament of God revealed "to us and our children forever."
At the same time, the hidden secrets of God are part of the Torah, secrets that are forever beyond us.
Nevertheless, the Torah serves as the witness "for" Israel, as well as, in the previous sense, serving to make Israel witness to the world.
–Rabbi Adin Steinsaltz
From "Sanctity and Restraint" in The Candle of God by Rabbi Adin Steinsaltz