already be there, because it is a condition of such perception. The one genuine proof that we are conscious of the law of causalty before all experience lies in the necessity of making a transition from the sensation, which is only empirically given, to its cause, in order that it may become perception of the external world. Therefore I have substituted this proof for the Kantian, the incorrectness of which I have shown. A most full and thorough exposition of the whole of this important subject, which is only touched on here, the a priori nature of the law of causality and the intellectual nature of empirical perception, will be found in my essay on the principle of sufficient reason, § 21, to which I refer, in order to avoid the necessity of repeating here what is said there. I have also shown there the enormous difference between the mere sensation of the senses and the perception of an objective world, and discovered the wide gulf that lies between the two. The law of causality alone can bridge across this gulf, and it presupposes for its application the two other forms which are related to it, space and time. Only by means of these three combined is the objective idea attained to. Now whether the sensation from which we start to arrive at apprehension arises through the resistance which is suffered by our muscular exertion, or through the impression of light upon the retina, or of sound upon the nerves of the brain, &c. &c., is really a matter of indifference. The sensation always remains a mere datum for the understanding, which alone is capable of apprehending it as the effect of a cause different from itself, which the understanding now perceives as external, i.e., as something occupying and filling space, which is also a form inherent in the intellect prior to all experience. Without this intellectual operation, for which the forms must lie ready in us, the perception of an objective, external world could never arise from a mere sensation within our skin. How can it ever be supposed that the mere feeling of being hindered in intended motion, which
