In Smell, as in the other senses, it is necessary to distinguish between the sensation itself and its object, which, in ordinary language, are not unfrequently confounded together. Thus we speak of the organ of smell, and of the smell of a rose, using the same term indifferently to signify the act of inhaling an odour and the odour inhaled.
The act of smell, apart from the physiological inquiries connected with it, requires no description, being familiar to every one from his own experience. It will be sufficient for our present purpose to distinguish the sensation from the accompanying perception; and this will be best accomplished by an examination of the object.
The true object of smell is to be found in the odorous particles in contact with the organ. It is incorrect to say that we smell a rose, meaning by the rose the flower as seen or touched. We smell only the effluvia emanating from the rose and coming in contact with the nervous organism. What these effluvia or odorous emanations are in themselves, natural philosophy is unable to determine.
Hence it is incorrect to speak, as Aristotle and many subsequent writers have spoken, of the object of smell as perceived through a medium, such as the atmosphere. The atmosphere is not the medium of communication between the sensitive organ and its object, but only the vehicle by which the object is brought into contact with the organ. Smell conveys to us no knowledge of the existence of extra-organic matter.
The only matter of which we are directly conscious in this, as in other actions of sense, is our own organism as extended; and this consciousness constitutes the perception of smell, as the consciousness of the same organism as affected constitutes the sensation. In the remarks upon the consciousness of space as the form of all sensitive intuition, enough has been said to explain in what sense the knowledge of locality and extension forms part of the energy of smell as it actually exists, and to show that the lowest degree of intelligence that is sufficient for sensation proper is sufficient for perception also.
But the object perceived is not a quality of body as such, but only the proper action, or rather passion, of our nervous organism; an action or passion of which the external cause is, so far as perception is concerned, wholly unknown, and which may even be excited in a similar manner by totally different causes. This is not so evident in the case of smell as in some others of the senses; yet it is a known fact in physiology that the sensation of smell may be produced in the olfactory nerve by electrical action without the presence of any odorous body.
This fact is sufficient to show that the operation of a sense by itself does not afford any legitimate grounds for determining the qualities, or even the existence, of an extra-organic world. On this subject we shall have more to say when we come to treat of the distinction between the primary and secondary qualities of body.
***Excerpt from Henry Longueville Mansel, B.d.. Metaphysics or the philosophy of consciousness