What is a relation?

Each relation has some elements, each element usually has a role within that relation. Moreover, there is a function, which takes this elements and creates new elements. A relation finds items of certain kinds (their roles), and creates other items of certain kinds, maybe even with a particular value

For example, in NUMBO:
Multiplication: item1 (operand) item2 (operand) item3 (result)

COPYCAT:
SUCESSOR: item1 (letter_value) item2 (Letter_Value) Alphabetic_Distance(Item1,Item2)=1 (number);

CHESS: Attack: item1(piece) item2(piece) move_distance(item1,item2)=1(number) (attacker in item1) (attacked in item2)

This is why I've found the quote from DeMorgan so sinister.