The distinction between properties and relations is an important one. Things possess properties singly. The fact that my pen is ten centimeters long is a fact about it and nothing else. On the other hand, relations involve pluralities of things. If the pen is on the desk then this situation involves both the pen and the desk. Binaly relations involve two things, ternary relations three things, and so on. Of course, everyting has many properties and stands in many relations to other things. (pp. 51-52)
Michell, J. (1990). An introduction to the logic of psychological measurement. Hillsdale, NJ: LEA.