“A second law of our nature,” says Dr Reid, “regarding perception is, that we perceive no object, unless some impression is made upon the organ of sense, either by the immediate application of the object, or by some medium which passes between the object and the organ. In two of our senses, to wit, _touch_ and _taste_, there must be an immediate application of the object to the organ. In the other three, the object is perceived at a distance, but still by means of a medium, by which some impression is made upon the organ. The effluvia of bodies drawn into the nostrils, with the breath, are the medium of smell; the undulations of the air, are the medium of hearing; and the rays of light passing from visible objects to the eye, are the medium of sight. We see no object, unless rays of light come from it to the eye. We hear not the sound of any body, unless the vibrations of some elastic medium, occasioned by the tremulous motion of the sounding body, reach our ear. We perceive no smell, unless the effluvia of the smelling body enter into the nostrils. We perceive no taste, unless the sapid body be applied to the tongue, or some part of the organ of taste. Nor do we perceive any tangible quality of a body, unless it touch the hands, or some part of our body.”[71] It is evident, that, in these cases of a supposed medium, which Dr Reid considers as forming so important a distinction of our sensations, the real object of _sense_ is not the _distant_ object, but that which acts immediately upon the organs,--the _light_ itself, not the _sun_ which beams it on us,--the odorous particles, which the wind has wafted to us from the rose, not the rose itself upon its stem,--the vibrations of the air, within our ear, not the cannon that is fired at the distance of miles. The light, the odour, the vibrating air, by which alone our senses are affected, act on our nerves of sight, of smell, and hearing, with an influence as direct, and as little limited in the kind of action, as that with which the fruit, which we eat or handle, acts on our nerves of taste or touch. This influence of the objects immediately external is all, in which our organs of sense, and consequently the mind as the principle of mere sensation, is concerned. The reference to the distant sun, or rose, or cannon, which alone leads us to speak of a medium in any of these cases, is the effect of _another_ principle of our intellectual nature,--the principle of association, or suggestion,--that is afterwards to be considered by us, without which, indeed, our mere transient sensations would be comparatively of little value; but which, as a quality or susceptibility of the mind, is not to be confounded with that, by which the mind becomes, instantly sentient, in consequence of a certain change produced in the state of its sensorial organ.