is sine plica, without a fold; as duplicity is duplex plica, a double fold. Conduct “without a fold” is straightforward, but thought without a fold is mere childishness. It is “tortuity of thought” that constitutes philosophic wisdom, and “simplicity of thought” that prepares the mind for faith.
“The flat simplicity of that reply was admirable.” —Vanbrugh and Cibber: The Provoked Husband, i.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894