Sentence:  The Yang , or male principle , <WSD>was</WSD> the source of light , heat , and dynamic vitality , associated with the Sun ; while the Yin , or female principle , flourished in darkness , cold , and quiet inactivity , and was associated with the Moon . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)