„Seek simplicity and distrust it.“

"Likeness to God", an address in Providence, Rhode Island (1828) http://www.americanunitarian.org/likeness.htm
Kontext: I affirm, and would maintain, that true religion consists in proposing, as our great end, a growing likeness to the Supreme Being. Its noblest influence consists in making us more and more partakers of the Divinity. For this it is to be preached. Religious instruction should aim chiefly to turn men's aspirations and efforts to that perfection of the soul, which constitutes it a bright image of God. Such is the topic now to be discussed; and I implore Him, whose glory I seek, to aid me in unfolding and enforcing it with simplicity and clearness, with a calm and pure zeal, and with unfeigned charity.

The Concept of Nature (1919), Chapter VII, p.143 http://www.gutenberg.org/files/18835/18835-h/18835-h.htm#CHAPTER_VII.
1910s
Kontext: The aim of science is to seek the simplest explanations of complex facts. We are apt to fall into the error of thinking that the facts are simple because simplicity is the goal of our quest. The guiding motto in the life of every natural philosopher should be, "Seek simplicity and distrust it."

A Conversation with Ward Cunningham (2003), The Simplest Thing that Could Possibly Work

C’est parce que la simplicité, parce que la grandeur est belle, que nous rechercherons de préférence les faits simples et les faits grandioses, que nous nous complairons tantôt à suivre la course gigantesque des astres, tantôt à scruter avec le microscope cette prodigieuse petitesse qui est aussi une grandeur, tantôt à rechercher dans les temps géologiques les traces d’un passé qui nous attire parce qu’il est lointain.
Part I. Ch. 1 : The Selection of Facts, p. 23
Science and Method (1908)

Quelle: Dictionary of Burning Words of Brilliant Writers (1895), P. 545.

Quelle: The Selfish Gene

Quelle: Walden

Letter to Coleridge 1790.

JPod (2006)
Quelle: The Gum Thief

Quelle: The Grand Design

1970s, How do we tell truths that might hurt? (1975)

Small Houses: Their Economic Design and Construction (1922)
Kontext: Simplicity and Dignity are so nearly related that they may be considered together.... A quiet air of reserved power is characteristic of dignity, and that is best obtained by simple means and the absence of apparent effort. Simplicity is the mark of genius. The giant in art does his work easily, without straining and without affectation; his ways are direct and to the point.

Quoted in The Routledge Dictionary of Latin Quotations (2005) by Jon R. Stone, p. 188
Spoken by Hus as he was being burned at the stake and saw an elderly peasant adding wood to the fire

No published occurrence of such an attribution has yet been located prior to one in Wisdom Through the Ages : Book Two (2003) by Helen Granat, p. 225; this was used as an early slogan at Apple Computer in 1984, but the earliest occurence yet located is in The Recognitions (1955) by William Gaddis, p. 457:
Stop being so God Damn humble … You know God damn well that … that humility is defiance … simplicity today is sophisticated … simplicity is the ultimate sophistication today.
Disputed

Quelle: The Fractalist (2012), Ch. 29, p. 299

Quelle: Dictionary of Burning Words of Brilliant Writers (1895), P. 545.
Quelle: The Imitation of Christ
Kontext: Simplicity and purity are the two wings by which a man is lifted above all earthly things. Simplicity is in the intention — purity in the affection. Simplicity tends to God,— purity apprehends and tastes Him.

A Voice from the Attic (1960)