No knowledge is possible about described things which do not have at least one property that is true of it.
That which has no property does not exist.
If there is no property that is true about x, then ~(x exists).
There is at least one property which is true of x, means, x exists.
(x exists) <-> EF(Fx).
~(x exist) <-> ~EF(Fx).
We cannot know about that which has no property.
