Keenness, n.
The quality or state of being keen.
Shades of meaning
Fernald has no entry of its own for keenness; he lists it under acumen.
Where he writes about it:
Shrewdness is keenness or sagacity, often with a somewhat evil bias, as ready to take advantage of duller intellects. — from acumen
We speak of the acuteness of an observer or a reasoner, the insight and discernment of a student, a clergyman, or a merchant, the sagacity of a hound, the keenness of a debater, the shrewdness of a usurer, the penetration, perspicacity, and acumen of a philosopher. — from acumen
The astute mind adds to acuteness and keenness an element of cunning or finesse. — from astute