Personal knowledge (with a specific subject etc.).
A state of being acquainted with a person; originally indicating friendship, intimacy, but now suggesting a slight knowledge less deep than that of friendship; acquaintanceship.
Such people collectively; one's circle of acquaintances (with plural concord).
A person or persons with whom one is acquainted.
Any science involved in studying phenomena or laws of the physical world: physics, chemistry, biology, astronomy, and so on.