Freud borrowed this term from Georg Groddeck's (1923) The Book of the It. Groddeck defines it thus:

I hold the view that man is animated by the Unknown, that there is within him an "Es," an "It," some wondrous force which directs both what he himself does, and what happens to him. The affirmation "I live" is only conditionally correct, it espresses only a small and superficial part of the fundamental principle, "Man is lived by the It."

(Groddeck, 1923/1961, p. 11)

The notion that we experience as other, as it rather than I, our own deepest sexual and aggressive motives -- and their linkage to memory images, to the flow of speech and action, and to the general tone of our personality -- is at the very center of Freud's psychology. His own best discussion of these matters is in the 1933 New Introductory lectures on Psychoanalysis, where Freud sums up the goal of therapy -- and indeed of all healthy personality development -- with the evocative epigram, "Where id was, there shall ego be" (Wo Es war, soll Ich werden, literally "Where it was, I shall come to be").