For the pure consideration of the nature of this object such circumstances are at first only a source of interference, because they behave as efficient causes and therefore make it uncertain whether specific alterations of colors, specific transitions and relations, are based on their own specific nature as color or are rather to be attributed to the specific pathology of the circumstances themselves, to the healthy and sick particular affections and effects of the organs of the subject, or to the chemical, vegetable, and animal forces of the objects.