First Principles

And here let us not omit to mark how that to which our intelligence is confined, is that with which alone our intelligence is concerned. The knowledge within our reach, is the only knowledge that can be of service to us. This maintenance of a correspondence between internal actions and external actions, which both constitutes our life at each moment and is the means whereby life is continued through subsequent moments, merely requires that the agencies acting upon us shall be known in their co-existences and sequences, and not that they shall be known in themselves. If _x_ and _y_ are two uniformly connected properties in some outer object, while _a_ and _b_ are the effects they produce in our consciousness; and if while the property _x_ produces in us the indifferent mental state _a_, the property _y_ produces in us the painful mental state _b_ (answering to a physical injury); then, all that is requisite for our guidance, is, that _x_ being the uniform accompaniment of _y_ externally, _a_ shall be the uniform accompaniment of _b_ internally; so that when, by the presence of _x_, _a_ is produced in consciousness, _b_, or rather the idea of _b_, shall follow it, and excite the motions by which the effect of _y_ may be escaped. The sole need is that _a_ and _b_ and the relation between them, shall always answer to _x_ and _y_ and the relation between them. It matters nothing to us if _a_ and _b_ are like _x_ and _y_ or not. Could they be exactly identical with them, we should not be one whit the better off; and their total dissimilarity is no disadvantage to us.