Full Idea
There was a prejudice against relations (in favour of properties) but De Morgan and others that impeccable inferences turn on relations and elude Aristotle's syllogistic. Thus: All horses are animals. Hence, all heads of horses are heads of animals.
Gist of Idea
De Morgan found inferences involving relations, which eluded Aristotle's syllogistic
Source
report of Augustus De Morgan (On the Syllogism IV [1859]) by William D. Hart - The Evolution of Logic 4
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.94
A Reaction
This is actually an early example of modern analytic philosophy in action. You start with the inferences, and then work back to the ontology and the definition of concepts. But in pinning down such concepts, do we miss their full meaning?