Why did Locke consider sensitive knowledge less certain than intuitive and demonstrative knowledge?

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Four colored straws in a glass of water appear bent at the water's surface, visually demonstrating the refraction of light. — Epistemology The refraction (bending) of light as it passes from air into water causes an optical illusion: straws in the glass of water appear broken or bent at the water's surface.

In his work An Essay Concerning Human Understanding, John Locke categorized knowledge into three degrees: intuitive, demonstrative, and sensitive. While he considered intuitive and demonstrative knowledge to be certain, he regarded sensitive knowledge—the perception of the particular existence of finite beings outside of ourselves—as less certain than the other two.1

Locke’s classification is based on the nature of how the mind perceives the agreement or disagreement of ideas. Intuitive knowledge is the most certain because the mind perceives this connection immediately, without the need for any other ideas. Demonstrative knowledge, while also certain, requires the mind to use mediating ideas and logical steps to perceive the connection, which demands effort and attention. Sensitive knowledge, however, is distinct because it concerns the existence of external objects. Because it relies on the senses to perceive these finite beings, it lacks the immediate or logically mediated necessity found in intuitive and demonstrative knowledge, placing it at a lower level of certainty in his epistemological framework.1