Rousseau (in _Emile_, Bk. II) regarded smell as the sense of the
imagination. So, also, at an earlier period, it was termed
(according to Cloquet) by Cardano. Cloquet frequently insisted on
the qualities of odors which cause them to appeal to the
imagination; on their irregular and inconstant character; on
their power of intoxicating the mind on some occasions; on the
curious individual and racial preferences in the matter of odors.
He remarked on the fact that the Persians employed asafoetida as
a seasoning, while valerian was accounted a perfume in antiquity.
(Cloquet, _Osphresiologie_, pp. 28, 45, 71, 112.) It may be
added, as a curious example familiar to most people of the
dependence of the emotional tone of a smell on its associations,
that, while the exhalations of other people's bodies are
ordinarily disagreeable to us, such is not the case with our own;
this is expressed in the crude and vigorous dictum of the
Elizabethan poet, Marston, "Every man's dung smell sweet i' his
own nose." There are doubtless many implications, moral as well
as psychological, in that statement.

The modern authorities on olfaction, Passy and Zwaardemaker, both
alike insist on the same characteristics of the sense of smell:
its extreme acuity and yet its vagueness. "We live in a world of
odor," Zwaardemaker remarks (_L'Annee Psychologique_, 1898, p.
203), "as we live in a world of light and of sound. But smell
yields us no distinct ideas grouped in regular order, still less
that are fixed in the memory as a grammatical discipline.
Olfactory sensations awake vague and half-understood perceptions,
which are accompanied by very strong emotion. The emotion
dominates us, but the sensation which was the cause of it remains
unperceived." Even in the same individual there are wide
variations in the sensitiveness to odors at different times, more
especially as regards faint odors; Passy (_L'Annee
Psychologique_, 1895, p. 387) brings forward some observations on
this point.

Maudsley noted the peculiarly suggestive power of odors; "there
are certain smells," he remarked, "which never fail to bring back
to me instantly and visibly scenes of my boyhood"; many of us
could probably say the same. Another writer (E. Dillon, "A
Neglected Sense," _Nineteenth Century_, April, 1894) remarks that
"n