  “[T]here appears to be a real foundation for the distinction; and it is this—that our senses give us a direct and a distinct notion of the primary qualities, and inform us what they are in themselves. But of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only, that they are qualities that affect us in a certain manner—that is, produce in us a certain sensation; but as to what they are in themselves, our senses leave us in the dark.” (EIP 2.17, 201)