arcane

vaikeaselkoinen, salainen understood by only a few
obscure, mysterious
requiring secret or mysterious knowledge to understand.
1997: Chris Horrocks, Introducing Foucault, page 67, The Renaissance Episteme (Totem Books, Icon Books; ISBN 1840460865)

A “signature” was placed on all things by God to indicate their affinities — but it was hidden, hence the search for arcane knowledge. Knowing was guessing and interpreting, not observing or demonstrating.