“Seek simplicity and distrust it.”
The Concept of Nature (1919), Chapter VII, p.143 http://www.gutenberg.org/files/18835/18835-h/18835-h.htm#CHAPTER_VII.
1910s
Context: 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."
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Alfred North Whitehead 112
English mathematician and philosopher 1861–1947Related quotes

"Likeness to God", an address in Providence, Rhode Island (1828) http://www.americanunitarian.org/likeness.htm
Context: 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
Context: 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."

“What is simplicity? Simplicity is the shortest path to a solution.”
A Conversation with Ward Cunningham (2003), The Simplest Thing that Could Possibly Work

“… the only simplicity to be trusted is the simplicity to be found on the far side of complexity.”

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)

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