In A Treatise concerning the Principles of Human Knowledge, Berkeley repudiates Locke’s theory of human perception. Where Locke argued that ideas come from one’s experience of an external, material world, Berkeley argued that the world itself is composed only of ideas. Berkeley held that ideas can only resemble other ideas: an idea in the human mind can only resemble an idea in the external world, not a material object. Consequently, Berkeley argued, being necessitates perception by a perceiver.
Going further, Berkeley argued that it is impossible to prove the existence of material objects external to the self, since all knowledge comes through one’s senses and therefore gives only knowledge of those senses. He argued that Locke’s assertion that primary qualities exist in abstraction, and are therefore knowable only through secondary qualities, was mistaken. It is only possible to know the qualities that are immediately perceptible to the human mind. If they are not directly perceptible (as Locke’s primary qualities were), then there is no way to know them and, therefore, we cannot say that they exist.
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