Sentence:  Having ſhewn how the <WSD>"Æther</WSD>" cauſes a great part of the Phænomena of Nature, it may be aſked, whence this general material Cauſe has its great Activity and Power? ... This Cauſe muſt be either Matter or Spirit, there being nothing in the Univerſe, which we know if, beſides theſe two. But this Cauſe cannot be Matter: for Matter is in own Nature inert, and has not any Activity in itſelf; and conſequently cannot communicate any Power to the <WSD>"Æther</WSD>". And therefore the Cauſe, which gives the <WSD>"Æther</WSD>" its Activity and Power, muſt be Spirit. Spirit, which intercedes the Particles of <WSD>"Æther</WSD>", and gives them a repulſive Power, and ordains and executes the Laws, by which <WSD>"Æther</WSD>" and Bodies act mutually on one another, muſt be preſent in all Parts of Space, where there is <WSD>"Æther</WSD>". 
Meaning of the word marked with <WSD> tags:  "Often as" '''(aether)''' "and more fully as" '''(luminiferous aether)''': a substance once thought to fill all unoccupied space that allowed electromagnetic waves to pass through it and interact with matter, without exerting any resistance to matter or energy; its existence was disproved by the 1887 (Michelson–Morley experiment) and the theory of relativity propounded by (Albert Einstein) (1879–1955).