May grandfather has a clearly defined birthday, which you might be oblivious to. This is imperfect knowledge.
You don’t know what I will have for lunch some day next week. -> This is unknown for entirely different reasons (!) -> To the extend that relevant choices have not yet been made this fact is not merely unknown, but Unknowable at the present time
Possible Objections
Some people may entertain the point of view that everything is purely mechanical, like a large machine or Billiard Table where in principle everything could be known, if only we could know every detail precise enough.
This idea is an over-idealized fantasy and is untenable due to some very basic properties of physical reality.
1: Quantum-Mechanical Uncertainty
Due to our physical realities we can only know the exact position of a body, or its velocity, yet not both at the same time.
This is known as Heisenberg Uncertainty and is a fundamental limitation of the world as we know it.
The mathematical operation called Fourier Transformation closely relates to it.
2: Controlling only 1 out of 3 dimensions
Any time we make a measurement we pick an axis.
Since we exist in three dimensions, this means any time we pick a direction for measurement we give up the entire perpendicular plane.
In other words: Since the choice on any variable always introduces two new ones, the total variety is growing faster than we can define, shape and select it.
The Universe of today simply isn’t big enough to contain all its possible futures (from a materialistic point of view).
Local Hidden Variables have been ruled out.
Jesse Michels points out how this looks a lot like a Caching Function where you can sample any particular point of view (axis of measurement) while never being able to ‘fix’ the deeper reality (i.e. knowing it with finality).
This results in the fascinating consequence that Measurement Selection is Fact Creation
A very interesting discussion of this feature is presented by Henry Strapp in a Panel Discussion on Free Will (Together with Donald Hoffman at SAND).
See also Delayed Choice Quantum Eraser, 2022 Nobel Prize in Physics by Greenberger, Horne and Zeilinger, Sensitivity to Initial Conditions