Definition of "principia"

I do really think, that your book […] might become an institute, containing the principia of those laws of motion, by which the system of the human community is framed and doth act […]

1776, Thomas Pownall, in a letter to the economist Adam Smith

They were her principia, her rudiments; the elementary atoms; the little steps by which she pressed forward to perfection.

1833, Elia [pseudonym; Charles Lamb], “Barbara S⁠——”, in The Last Essays of Elia. […], London: Edward Moxon, […], page 116