It is an axiom in mental philosophy, that we can think of nothing which we have not perceived. When I say that we can think of nothing, I mean we can imagine nothing, we can reason of nothing, we can remember nothing, we can foresee nothing. The most astonishing combination of poetry, the subtlest deductions of logic and mathematics, are no other than combinations which the intellect makes of sensations according to its own laws. A catalogue of all the thoughts of the mind, and of all their possible modifications, is a cyclopaedic history of the universe.