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seem to be in it a direct knowledge of one force at least, that of light, alien and external to itself.

But even on this supposition, farther reflection would modify our conclusion. In purely physical causation, the cause, though entering into the effect, is not as a cause recognizable there. Indeed it seems probable that there is not invariably the same transferred force in one series of effects as in another, and that in some results the prime agency quickly disappears. A ball is struck by a bat and get in motion: after the ball has parted from the bat how much of the antecedent fact could be found in the subsequent one of independent motion? How far would the second phenomenon directly disclose the first, or what common term or force could be detected in the two? The force is not discernible aside from the results it occasions, and antecedent effects are not given in subsequent ones. Suppose the same ball to be observed falling under the influence of gravitation. How far would this new cause be discoverable directly in this new phase of movement? Again, chemical action is initiated by a rise of temperature; water is instantly frozen under certain conditions by a slight jar; the brain is quickened by a full stomach; in these and a thousand other cases of causation, what portion of the cause is in the effect, to be found there as a part in a whole, as the numbers, 6, 3, 3, in the sum twelve. Evidently in a purely physical effect it is impossible for us to detect the cause as a cause as a second, alien agency, entering into and constituting a distinguishable part of the new, simple state before us.

We perceive phenomena only, not the underlying forces, not the very causes; these, and the antecedent facts they may have occasioned, are matters of inference and of experience exclusively. If, then, the phenomena transpiring in the eye were, as they are not, identical with those of the