Read another response by Richard Heck
Can we perceive relations? For example, if I have a cup of coffee I can perceive the cup as white, round, hard, and shiny; and the coffee as liquid, brown, hot, and delicious; but the relation in has no color or visual size or shape, and I cannot touch it, hear it, smell or taste it --- so how can I perceive it? It's tempting to say that I cannot perceive it because it isn't real --- but if it isn't real then how could I drink the coffee? The similarity between two oranges, the direction of a train whistle, the relative brightness of the sun and the full moon ... There are countless empirical relations that can/cannot be perceived. How come?