“The madman is a dreamer awake”

Source: The Shaking Woman, or A History of My Nerves

Source: I Have a Dream

David R. Slavitt, The Voyage of the Argo: The Argonautica of Gaius Valerius Flaccus (1999), Book I, lines 98–99 (p. 3). There is no corresponding text in the Latin (cf. Argon. 1.79–80).
Misattributed

As A Man Thinketh (1902), Visions and Ideals
Context: The dreamers are the saviours of the world. As the visible world is sustained by the invisible, so men, through all their trials and sins and sordid vocations, are nourished by the beautiful visions of their solitary dreamers. Humanity cannot forget its dreamers; it cannot let their ideals fade and die; it lives in them; it knows them as the realities which it shall one day see and know.

Ibid., p. 110
The Book of Disquiet
Original: A superioridade do sonhador consiste em que sonhar é muito mais prático que viver, e em que o sonhador extrai da vida um prazer muito mais vasto e muito mais variado do que o homem de acção. Em melhores e mais directas palavras, o sonhador é que é o homem de acção.

Preface.
A History of Science Vol.1 Ancient Science Through the Golden Age of Greece (1952)
Context: Some men are abstract-minded, and they naturally think first of unity and God, of wholeness, of infinity and other such concepts, while the minds of other men are concrete and they cogitate about health and disease, profit and loss. They invent gadgets and remedies; they are less interested in knowing anything than in applying whatever knowledge... to practical problems... The first are called dreamers; the second kind are recognized as practical and useful. History has often proved the shortsightedness of the practical men and vindicated the "lazy" dreamers; it has also proved that the dreamers are often mistaken.

Source: All the Light We Cannot See