Full Idea
If there were a vacuum in space, one could establish its size. But if there were a vacuum in time, i.e. a duration without change, it would be impossible to establish its length.
Gist of Idea
If there were duration without change, we could never establish its length
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.15)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.155
A Reaction
See Idea 4226 for Shoemaker's wonderful counterproposal to this apparently unanswerable claim. I suppose Leibniz is right, but it just might be possible to bring induction to bear on the problem.
Related Idea
Idea 4226 If three regions 'freeze' every three, four and five years, after sixty years everything stops for a year [Shoemaker, by Lowe]