Full Idea
If the ratio of two lines L and M is conceived as abstracted from them both, without considering which is the subject and which the object, which will then be the subject? We cannot say both, for then we should have an accident in two subjects.
Gist of Idea
The ratio between two lines can't be a feature of one, and cannot be in both
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], 5th Paper, §47), quoted by John Heil - Relations 'External'
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.315
A Reaction
[compressed] Leibniz is rejecting external relations as having any status in ontology. It looks like a mistake (originating in Aristotle) to try to shoehorn the ontology of relations into the substance-properties framework.
Related Idea
Idea 21345 Aristotle said relations are not substances, so (if they exist) they must be accidents [Aristotle, by Heil]