James Strachey writes in a note to his edition of The Interpretation of Dreams, " 'If I cannot bend the Higher Powers, I will move the Infernal Regions.' Freud remarks in a note . . . (1925) . . . that 'this line of Virgil . . . is intended to picture the efforts of the repressed instinctual impulses.' He has used the same line as the motto for the whole volume. In a letter to Fleiss of December 4, 1896 . . . he proposed using it as a motto for a chapter on 'Symptom Formation' in some projected but unrealized work" (647).
In The Interpretation of Dreams, Freud refers to Joseph's method of dream interpretation as one of two popular (and hence incorrect) methods of dream-interpreting which he calls "symbolic": "[This procedure] considers the content of the dream as a whole and seeks to replace it by another content which is intelligible and in certain respects analogous to the original one. . . . [But this procedure] inevitably breaks down when faced by dreams which are not merely unintelligible but also confused" (129).
His example of this "symbolic" procedure? Joseph's interpretation of Pharaoh's dream:
The seven fat kine followed by seven lean kine that ate up the fat kine--all this was a symbolic substitute for a prophecy of seven years of famine in the land of Egypt which should consume all that was brought forth in the seven years of plenty. Most of the artificial dreams constructed by imaginative writers are designed for a symbolic interpretation of this sort: they reproduce the writer's thoughts under a disguise which is regarded as harmonizing with the recognized characteristics of dreams. The idea of dreams being chiefly concerned with the future and being able to foretell it--a remnant of the old prophetic significance of dreams--provides a reason for transposing the meaning of the dream, when it has been arrived at by symbolic interpretation, into the future tense. (129)
This passage, regarding Joseph's method of dream-interpreting, in fact provides us with a way of understanding what appears to have been the figurative way Freud read the line from Virgil, which became his life-long motto:
Flectere si nequeo superos, Acheronta movebo
First, Freud dismissed dreams as prophetic (as being concerned with the "future tense") and in doing so removed their divine origin--a divine warning of some impending disaster (though certainly he was interested in intuitive premonitions and moments of deja vu and jamais vu which he called "unheimlich" or the uncanny. We can read "Flectere si nequeo superos" ("Since, or if, I am unable to bend or 'flex' the higher, or superior beings' will") as an implicit indication of dreams' divine origin. He then replaced the cosmological and prophetic significance of dreams with a psychological register--"Acheronta movebo" ("I shall or will move, or perhaps 'lift up' or 'raise,' the infernal regions [Acheronta]"). This infernal region of the psyche he (re)named the unconscious.