Sentence:  Of them the other philosophers have, by <WSD>siths</WSD>, taken their beginning.<!-- Some notes: "Secretum Secretorum in Ashmole 396", Bodelian Library, Oxford, Elias Ashmole, "Of them the other philosophres..have be sithe toke thair begynnyng." per the Middle English Dictionary. --> 
Meaning of the word marked with <WSD> tags:  An instant in time, a point in time or an occasion.