Why did John Locke consider much scientific knowledge probable rather than certain?

John Locke defined knowledge as the perception of the connection and agreement—or disagreement—between ideas. Under this strict definition, he argued that scientific knowledge, excluding mathematics, generally falls into the category of probability rather than certainty.123

Locke maintained that scientific claims lack the necessary "evident connection" between ideas required for genuine knowledge. For instance, while one can intuitively know that white is not black, one cannot know with absolute certainty that a dropped stone will fall or that water will quench thirst. Although humans possess a strong psychological expectation that these events will occur through the association of ideas, these observations do not constitute certain knowledge because they lack an immediate, demonstrable link between the ideas involved.13

For Locke, scientific understanding is based on experience and observation, which he categorized as probability rather than the intuitive or demonstrative certainty found in logic or mathematics. While he acknowledged that these probable claims are essential for navigating the world, he insisted they remain distinct from the higher standard of certain knowledge.321