" Constitution and Heredity.—In the first place, we must mention here the congenital variation of the sexual constitution, upon which the greatest weight probably falls, but the existence of which, as may be easily understood, can be established only through its later manifestations and even then not always with great certainty. We understand by it a preponderance of one or another of the manifold sources of the sexual excitement, and we believe that such a difference of disposition must always come to expression in the final result, even if it should remain within normal limits. "

― Sigmund Freud , The Collected Works of Sigmund Freud: The Complete Works PergamonMedia (Highlights of World Literature)