Aristotle seems to have been tolerably satisfied with the above explanation of sight and hearing; for, in approaching the sense of Smell with the olfacients, he begins by saying that it is less definable and explicable. Among the five senses, smell stands intermediate between the two (taste and touch) that operate by direct contact, and the other two (sight and hearing) that operate through an external medium. Man is below other animals in this sense; he discriminates little in smells except the pleasurable and the painful.[88] His taste, though analogous in many points to smell, is far more accurate and discriminating, because taste is a variety of touch; and in respect to touch, man is the most discriminating of all animals. Hence his great superiority to them in practical wisdom. Indeed the marked difference of intelligence between one man and another, turns mainly upon the organ of touch: men of hard flesh (or skin) are by nature dull in intelligence, men of soft flesh are apt and clever.[89] The classifying names of different smells are borrowed from the names of the analogous tastes to which they are analogous — sweet, bitter, tart, dry, sharp, smooth, &c.[90] Smells take effect through air as well as through water; by means of a peculiar agency or accompaniment (mentioned above, called the Trans-Olfacient) pervading both one and the other. It is peculiar to man that he cannot smell except when inhaling air in the act of inspiration; any one may settle this for himself by making the trial.[91] But fishes and other aquatic animals, which never inhale air, can smell in the water; and this proves that the trans-olfacient agency is operative to transmit odours not less in water than in air.[92] We know that the sense of smell in these aquatic animals is the same as it is in man, because the same strong odours that are destructive to man are also destructive to them.[93] Smell is the parallel, and in a certain sense the antithesis of taste; smell is of the dry, taste is of the moist: the olfactory matter is a juicy or sapid dryness, extracted or washed out from both air and water by the trans-olfacient agency, and acting on the sensory potentialities of the nostrils.[94] This olfactory inhalation is warm as well as dry. Hence it is light, and rises easily to the brain, the moisture and coldness of which it contributes to temper; this is a very salutary process, for the brain is the wettest and coldest part of the body, requiring warm and dry influences as a corrective. It is with a view to this correction that Nature has placed the olfactory organ in such close proximity to the brain.[95] There are two kinds of olfactory impressions. One of them is akin to the sense of taste — odour and savour going together — an affection (to a great degree) of the nutritive soul; so that the same odour is agreeable when we are hungry, disagreeable when our hunger is fully satisfied. This first kind of impression is common to men with other animals; but there is a second, peculiar to man, and disconnected from the sense of taste, viz., the scent of flowers, unguents, &c., which are agreeable or disagreeable constantly and per se.[96] Nature has assigned this second kind of odours as a privilege to man, because his brain, being so large and moist, requires to be tempered by an additional stock of drying and warming olfactory influence.
[88] Aristot. De Animâ, II. ix. p. 421, a. 7. De Sensu et Sensili, v. p. 445, a. 6; iv. p. 441, a. 1. De Partibus Animal. II. xii. p. 656, a. 31; p. 657, a. 9.
[89] Aristot. De Animâ, II. ix. p. 421, a. 21: κατὰ δὲ τὴν ἁφὴν πολλῷ τῶν ἄλλων ζῴων διαφερόντως ἀκριβοῖ (ὁ ἄνθρωπος). διὸ καὶ φρονιμώτατόν ἐστι τῶν ζῴων. σημεῖον δὲ τὸ καὶ ἐν τῷ γένει τῶν ἀνθρώπων παρὰ τὸ αἰσθητήριον τοῦτο εἶναι εὐφυεῖς καὶ ἀφυεῖς, παρ’ ἄλλο δὲ μηδέν· οἱ μὲν γὰρ σκληρόσαρκοι ἀφυεῖς τὴν διάνοιαν, οἱ δὲ μαλακόσαρκοι εὐφυεῖς.
[90] Ibid. a. 26.
[91] Ibid. b. 9-19. τὸ ἄνευ τοῦ ἀναπνεῖν μὴ αἰσθάνεσθαι ἴδιον ἐπὶ τῶν ἀνθρώπων· δῆλον δὲ πειρωμένοις. He seems to think that this is not true of any animal other than man.
[92] Aristot. De Sensu et Sensili, v. p. 443, a. 3-31; p. 444, b. 9.
[93] Aristot. De Animâ, II. ix. p. 421, b. 23. He instances brimstone, ἄσφαλτος, &c.
[94] This is difficult to understand, but it seems to be what Aristotle here means. — De Animâ, II. ix. p. 422, a. 6: ἔστι δ’ ἡ ὀσμὴ τοὺ ξηροῦ, ὥσπερ ὁ χυμὸς τοῦ ὑγροῦ· τὸ δ’ ὀσφραντικὸν αἰσθητήριον δυνάμει τοιοῦτον. — De Sensu et Sensili, v. p. 443, a. 1-9: ἔστι δ’ ὀσφραντὸν οὐχ ᾗ διαφανές, ἀλλ’ ᾖ πλυντικὸν ἢ ῥυπτικὸν ἐγχύμου ξηρότητος· — ἡ ἐν ὑγρῷ τοῦ ἐγχύμου ξηροῦ φύσις ὀσμή, καὶ ὀσφραντὸν τὸ πάθος, δῆλον ἐκ τῶν ἐχόντων καὶ μὴ ἐχόντων ὀσμήν, &c. Also p. 443, b. 3-7.
In the treatise De Sensu et Sensili, there is one passage (ii. p. 438, b. 24), wherein Aristotle affirms that smell is καπνώδης ἀναθυμίασις, ἐκ πυρός; but we also find a subsequent passage (v. p. 443, a. 21, seq.) where he cites that same doctrine as the opinion of others, but distinctly refutes it.
[95] Aristot. De Sensu et Sensili, v. p. 444, a. 10, 22, 24: ἡ γὰρ τῆς ὀσμῆς δύναμις θερμὴ τὴν φύσιν ἐστίν.