"The sensations and ideas of extension, as far as can be ascertained, are suggested at the very same instant," where, it may be observed, that he uses the very objectionable word " suggest." Almost the whole of the body, whether external or internal, is sensitive and an impression on every sensitive part, whether occasioned by an external or internal influence, is accompanied with a sensation." " It is also accompanied, I imagine.with an idea of extension corresponding to the form and magnitude of the impression."" We always find that an impression on the organ of taste is accompanied not only with a sensation, but with an idea of the part of the organ affected." " It is highly probable that the organ of smell also affords ideas of extension." "The sense of hearing seems to be governed by the same laws."Locke taught that we get our ideas of extension solely from sight and touch.Ballantyne maintains that we have them from all the senses, because by all the senses we have " an idea of the part of the organ affected," here anticipating favorite positions of Hamilton and the physiologist Muller.
He criticises the doctrine of Brown, that by the eye we discern only color, not in so climbing a manner as Hamilton, but in a like spirit.Some indeed contend that sight affords only sensations of color, and no idea of extension at all;but this opinion has never been established by adequate evidence and appears to be incompatible with not a few phenomena.At any rate, in the present state of our knowledge, we certainly have {391} by sight ideas of extension and ideas of greater or smaller portions of it, other things being equal, according to the impression on the organ, and are bound, therefore, to regard this as an ultimate principle, till it be traced to one more general.
This, so far as I know, has never yet been done, and I am utterly unable to perceive how it can be done."" We never but find that an impression on a sensitive organ is accompanied with an idea of duration, as well as with a sensation and an idea of extension." " Every impression, besides suggesting a sensation and an idea of extension, suggests in connection with them an idea of a portion of duration corresponding to the duration of the impression." And here he has again to criticise Brown, and by anticipation Mill." There are no doubt many analogies between duration and extension; but to assume that they are literally one and the same thing, as Dr.Brown most evidently does, is one of the most unwarrantable assumptions that ever was hazarded." "That doctrine is, that duration and extension are substantially one and the same thing, and that a cubical foot, or a cubical yard, is not essentially different from an hour or a day." The sensations and ideas of extension " will give rise to the notion of length of duration as occurring in different points of duration, and of course as occurring in succession; but, unless length of duration be the same thing with length of extension, they can evidently give us no manner of notion of this latter species of length at all."He has a very elaborate inquiry into the associative principle, "a branch of our constitution still involved in considerable obscurity." As I believe him to be right in affirming that at this point there are many unsettled questions, and as his observations are original and independent, I will quote from him at considerable length.