Isaac Newton Death Mask
Hypotheses non fingo (Latin for I feign no hypotheses) is a famous phrase used by Isaac Newton in an essay General Scholium which was appended to the third edition of the Principia.
It was his answer to those who had publicly challenged him to give an explanation for the causes of gravity rather than just the mathematical principles of kinetics. Along with Occam's Razor, the term can be seen as a departure from the Aristotelian concept of natural philosophy.
Ironically, in private Newton was obsessed with the causes of gravity, positing the existence of an aether in many of his unpublished works, to solve the action-at-a-distance problem. This problem was eventually changed to a different perspective on the same problem by Albert Einstein in the context of classical physics by the theory of General Relativity.
Here is a translation 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. 
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