In Matthew 12:31-32, this signifies to speak against the literal sense of the Word, or the divine truth as it is on earth; while to speak against the Holy Spirit is to utter blasphemies against the Divine Truth as it is in the Lord, or in heaven. (Apocalypse Explained 778[3])

Fowl signify spiritual truth; a bird, natural truth; and a winged thing, sensual truth. Fowl of heaven, as in Hosea 2:18, signify the affections of truth; and reptiles of the earth, the affection of the knowledges of truth and good. Fowl signify thoughts, and all that creeps on the ground, the sensual principle. Fowl signify intellectual things.

(références: Arcana Coelestia 777; Hosea 2)