Philosophy
The Metaphysics of Properties: Universals vs. Tropes
Quick fact
The debate between universals and tropes can be traced back to Plato and Aristotle, and it remains one of the most heated disputes in analytic metaphysics today.
Why this is interesting
When you say 'the rose is red' and 'the apple is red', what makes them both 'red'? Is 'redness' a thing that can be shared, or is each red object just its own miniature redness?