Isaac Newton — "The causes of gravity are not yet discovered. It is by experiments and observati…"

The causes of gravity are not yet discovered. It is by experiments and observations that we are to know them.
Isaac Newton — Isaac Newton Early Modern · Laws of motion and gravity

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From a letter to Richard Bentley

Date: 1692

General

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Understanding this quote

What it means

Newton admits he can describe precisely how gravity behaves mathematically but refuses to claim he knows what produces it. He insists the answer must come from real-world evidence—experiments and observations—not philosophical speculation. This separates measurable description from unverified explanation, a standard modern science still demands. Knowing how something works and knowing why it works are two entirely different questions, and Newton refused to blur that line without proof.

Relevance to Isaac Newton

Newton's famous Latin declaration 'hypotheses non fingo'—I frame no hypotheses—runs through the Principia Mathematica, his masterwork on motion and gravity. He derived universal gravitation with extraordinary precision yet consistently refused to speculate on its physical cause. This quote proves that restraint wasn't modesty but method: Newton separated what evidence demonstrated from what it couldn't yet reach, practicing rigorous intellectual discipline even while dismantling centuries of Aristotelian natural philosophy.

The era

In the 17th century, natural philosophers competed to explain gravity through metaphysical speculation—Descartes proposed swirling vortices; others invoked divine action. The Scientific Revolution was displacing Aristotelian authority, but theorizing without experimental proof remained acceptable. Newton's insistence that causes require observational confirmation directly challenged Cartesian mechanical philosophy and the broader culture of armchair cosmology. At a moment when claiming to know gravity's cause would have impressed peers, Newton's public admission of ignorance was quietly revolutionary.

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