 Two of Reid’s reflections on this contrast are especially important. First, there are no disputes about the natures of the primary qualities because the are exhibited to sense. Primary qualities are investigated by the a priori methods of the mathematical sciences. Reid attributes this to the distinctness of our concepts of them and the knowledge of their natures the concepts provide. They are subjects of conceptual necessary truths. But because secondary qualities are obscure to sense, they are an appropriate subject of the sort of formal inquiry conducted in natural philosophy. Their natures are not known directly by the senses but may be discovered by observation, reasoning, and deft use of hypotheses; e.g., we know vibrations cause sounds and reflected light causes sensations of colors (on the proper method of natural philosophical inquiry (see Reid 1785 [2002: 267–75]).