  As we saw in passages discussed earlier, in connection with the impossibility of scientia, Locke finds the production of sensation to be utterly obscure. One side of the difficulty is, of course, the nature of the mind. In all probability, it is immaterial. However, Locke allows, it is possible that God has superadded the power of thought directly to matter. The other side of the difficulty concerns the nature of secondary qualities as powers to produce sensations. The appeal of the corpuscular hypothesis lay largely in its reductive promise. Secondary qualities in particular, such as colors and sounds, but also ideas of macrolevel primary qualities, including visual sensations of shapes and sizes, and tertiary qualities were to be reduced to the primary qualities of corpuscles, interacting with one another and/or with perceptual systems.