Insight, n.
1. A sight or view of the interior of anything; a deep inspection or view; introspection; -- frequently used with into. He had an insight into almost all the secrets of state. Jortin.
2. Power of acute observation and deduction; penetration; discernment; perception. Quickest insight In all things that to greatest actions lead. Milton.
Shades of meaning
Fernald has no entry of its own for insight; he lists it under acumen, wisdom.
Where he writes about it:
Sharpness, acuteness, and insight, however keen, and penetration, however deep, fall short of the meaning of acumen, which implies also ability to use these qualities to advantage. — from acumen
There are persons of keen insight and great penetration to whom these powers are practically useless. — from acumen
Insight and discernment are applied oftenest to the judgment of character; penetration and perspicacity to other subjects of knowledge. — 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
Insight, judgment, profundity or depth, reason, sagacity, sense, and understanding are native qualities of mind, tho capable of increase by cultivation. — from wisdom