(1):

(n.) State of being dual or twofold; a twofold division; any system which is founded on a double principle, or a twofold distinction

(2):

(n.) The theory that each cerebral hemisphere acts independently of the other.

(3):

(n.) The doctrine that all mankind are divided by the arbitrary decree of God, and in his eternal foreknowledge, into two classes, the elect and the reprobate.

(4):

(n.) A system which accepts two gods, or two original principles, one good and the other evil.

(5):

(n.) A view of man as constituted of two original and independent elements, as matter and spirit.