Sentence:  This and other fears of the solar system have disappeared gradually , first , with the Ptolemaic system and its built-in <WSD>concept</WSD> of periodicity and then , more firmly , with the Newtonian innovation of an universal force that could account quantitatively for both terrestrial and celestial motions . 
Meaning of the word marked with <WSD> tags:  an abstract or general idea inferred or derived from specific instances