War is peace. Freedom is slavery. Ignorance is strength
05AB1E, 30 bytes

05AB1E uses CP-1252.

“ignorance¤í‡î—™šÔÃÒry“#DIk1^è


Try it online! or as a Test suite

Explanation

The straight forward approach


Push the string ignorance strength war peace freedom slavery
Split on spaces
Get the index of the input in the list
XOR the index with 1
Get the element in the list at that index