Sentence:  The concept of unity , in which positive and negative are attributes of the same force , in which good and evil are relative , ever-changing , and always joined to the same phenomenon - such a concept is still reserved to the physical sciences and to the few who have grasped the history of <WSD>ideas</WSD> . 
Meaning of the word marked with <WSD> tags:  the content of cognition; the main thing you are thinking about