 Hume distinguishes two kinds of impressions: impressions of sensation, or original impressions, and impressions of reflection, or secondary impressions. Impressions of sensation include the feelings we get from our five senses as well as pains and pleasures, all of which arise in us “originally, from unknown causes” (T 1.1.2.1/7). He calls them original because trying to determine their ultimate causes would take us beyond anything we can experience. Any intelligible investigation must stop with them.