While it’s true that when something underlies something else, it is beneath it, this does not necessarily mean subservience — it can also indicate a strong foundation or bedrock that underlies, or supports, an idea or decision.

The Old English word underlicgan meant to serve or submit to another, and that meaning does carry through to the modern word underlie — to a point. Although still referring to something under something else, around 1850 the meaning of the verb shifted to include something that makes a foundation for something else. For example, one factor that underlies a patient's unusual symptoms might be his recent trip to a foreign country, where he could have picked up a local infection.

Definitions of underlie

v lie underneath

Type of:
be located or situated somewhere; occupy a certain position

v be or form the base for

Type of:
have the quality of being; (copula, used with an adjective or a predicate noun)

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