وهو أن يكون المشبه به نادر الحضور في الذهن
That is, the comparison term is rarely present to the mind.
Rarity need not mean that the comparison term is physically scarce. Two individually familiar scenes may hardly ever be contemplated together as analogous. A poet can connect them in a previously unsuspected way, producing an unexpected mental association.
Thus distinguish external rarity from the mental rarity of a resemblance. Even common objects may yield an uncommon rhetorical comparison when their relation rarely occurs to the audience. The relationship must, however, become intelligible after reflection; unconnected oddity alone does not amount to an effective simile.