When Hume speaks about the distinction between cause and effect, he distinguishes between thoughts, ideas, and impressions. Impressions, he says, are the only true foundation and source of knowledge; they are the basis for all our ideas and every single thought we can have. There is nothing in the mind that was not first in the senses. All thoughts owe their existence to some experience. As Hume insists, “all our ideas or more feeble perceptions are copies of our impressions or more lively ones” (13).

One way that we can understand this is by comparing the experience of our impressions with our ideas. An idea is a purely mental reality; it is within the mind. We have ideas by pulling from our memories, since “every idea which we examine is copied from a similar impression” (14). When we remember a warm fire, or a delicious meal, we experience delight and good feelings, but the memory of them pales in comparison with the actual experience. In this way, we easily recognize the superiority of the experience over the idea.

The impression is superior and needs to be the source of all our knowledge.