Berkeley asserts that, although our perceptions seem to inform us of a world outside ourselves, the mind does not –cannot– experience such things directly, but only as Ideas. “Our sensations,” he writes, “be they never so vivid and distinct, are nevertheless Ideas.”
We are, says Berkeley, “unable to comprehend in what manner body can act upon Spirit, or how it is possible it should imprint any Idea in the mind.” This is why Berkeley contends that “the testimony of sense” is not enough to assume the reality of physical objects around us. “It is evident the production of Ideas or sensations in our minds can be no reason why we should suppose matter or corporeal substances.”
Consider dreams, says Berkeley… Dreams show us that “it is possible we might be affected with all the Ideas we have now, though there were no bodies existing without, resembling them. Hence it is is evident the supposition of external bodies is not necessary for the producing our Ideas.”
However, Berkeley does not push the dream analogy too far. He acknowledges the difference between the dreamworld and the waking world… In the waking world, “the Ideas of sense are more strong, lively, and distinct than those of the imagination.” When we are awake, our perceptions occur in a “regular train or series” and have “a steadiness, order, and coherence” that they do not possess in the dreamworld. Nevertheless, the dream analogy is worthy food for thought when one comes to consider the trustworthiness of the sources of our mental images and other sensations.