Qualities are almost always the objects of our external senses
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“First of all, a relation is, in itself, a more metaphysical object than a quality. Nobody can be at a loss to explain what is meant by a quality; but few people will find themselves able to express, very distinctly, what is understood by a relation. Qualities are almost always the objects of our external senses; relations never are.5 No wonder, therefore, that the one set of objects should be so much more comprehensible than the other. Secondly, though prepositions always express the relation which they stand for, in concrete with the co-relative object, they could not have originally been formed without a considerable effort of abstraction.” source