Hypotheses non fingo (Latin for "I feign no hypotheses", "I frame no hypotheses", or "I contrive no hypotheses") is a famous phrase used by Isaac Newton in an essay, "General Scholium", which was appended to the second (1713) edition of the Principia.
Here is a modern translation (published 1999) of the passage containing this famous remark:
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses. For whatever is not deduced from the phenomena must be called a hypothesis; and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy. In this philosophy particular propositions are inferred from the phenomena, and afterwards rendered general by induction.
The nineteenth century philosopher of science, William Whewell, qualified this statement, as, he said, "it was by such a use of hypotheses, that both Newton himself and Kepler, on whose discoveries those of Newton were based, made their discoveries". Whewell stated:
What is requisite is, that the hypotheses should be close to the facts, and not connected with them by other arbitrary and untried facts; and that the philosopher should be ready to resign it as soon as the facts refuse to confirm it.
Later, Imre Lakatos would assert that that resignation should not be too rushed.