Sentence:  Now, certainly, in a general view, the purpose of the Platonic soldier was the same, but with this important difference—that his fighting condition was not needed on one or two days consecutively, but on many days, and not against a day punctually <WSD>assignable,</WSD> but against a season or period perhaps of months, quite indeterminate as to its beginning, end, or duration. 
Meaning of the word marked with <WSD> tags:  Capable of being specified or shown.