One of the lesser used modalities is the olfactory system. For most people it is too unconscious to be of much use. Nevertheless, given that the olfactory system works so unconsciously when it works, it works really, really well. For that reason it is one of the systems to check if the main modalities, visual, auditory & kinestetic, don’t work as well as wished for.
And what magnificent instruments of observation we possess in our senses! This nose, for example, of which no philosopher has yet spoken with reverence and gratitude, is actually the most delicate instrument so far at our disposal: it is able to detect tiny chemical concentrations that even elude a spectroscope. Today we possess science precisely to the extent to which we have decided to accept the testimony of the senses — to the extent to which we sharpen them further, arm them, and have learned to think them through. The rest is miscarriage and not-yet-science — in other words, metaphysics, theology, psychology, epistemology — or formal science, a doctrine of signs, such as logic and that applied logic which is called mathematics. In them reality is not encountered at all, not even as a problem — no more than the question of the value of such a sign-convention as logic.
Twilight of the Idols, ‘Reason’ In Philosophy, paragraph 3