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force, occupying space, called matter. This permanent force is necessary to the notion of matter, and is as appropriately reached by one sense as by another, by one effect as by another; indeed, is indicated by any sensation which betrays an external world. The qualities which find entrance through one organ, have no more right to be called primary, that is fundamental, than those which enter at another. If the sense of muscular effort were wanting, we might still be able to arrive at the idea of matter; though its alleged primary quality should not be directly recognizable by us. We should then understand color and flavor as indications of a local force, apprehensible by sight and taste.

The second criterion more signally fails than the first in its application to solidity. Far from matter's coming most directly and fully in contact with mind through solidity, in many instances it is only in a secondary, inferential way, that this quality is at all arrived at. A gas makes no impression on the muscular system, offers no obstacle to movement, calls forth no sense of resistance, till closely confined; and then by that very confinement is put beyond direct contact with any organ of sense. We are left to infer the solidity of gases, from the fact that the piston cannot, in the cylinder containing them, be forced perfectly down to its bed, and recoils as the hand is lifted. Perception is not more immediate and full here than elsewhere; on the contrary, there is no perception of the point at issue, the solidity of the gas, but only a judgment to that effect. Even the solidity of a solid directly handled is inferred from the muscular effort expended in the attempt to crush it, and only admits of an estimate by an indirect method.

This doctrine of primary and secondary qualities, maintained by so large a variety of philosophers, is of interest, chiefly from the way in which it has grown out of the