"不是基於理論而是直接經驗和觀察的依賴性; " 最初用於醫學意義上的1650年代，來自 empiric 和 -ism。最初的醫學意義是貶低的：“江湖醫術; 無知者冒充有醫學技能。”這種貶低的特性隨後體現在“依賴於直接觀察而不是理論”的普遍意義上，特別是對單純個人經驗的過度依賴。至於哲學上將經驗視爲唯一知識來源的教義，始於1796年。
Were I obliged to give a short name to the attitude in question, I should call it that of radical empiricism, in spite of the fact that such brief nicknames are nowhere more misleading than in philosophy. I say 'empiricism' because it is contented to regard its most assured conclusions concerning matters of fact as hypotheses liable to modification in the course of future experience; and I say 'radical,' because it treats the doctrine of monism itself as an hypothesis, and, unlike so much of the half way empiricism that is current under the name of positivism or agnosticism or scientific naturalism, it does not dogmatically affirm monism as something with which all experience has got to square. The difference between monism and pluralism is perhaps the most pregnant of all the differences in philosophy. [William James, preface to "The Sentiment of Rationality" in "The Will to Believe and Other Essays in Popular Philosophy," 1897]
如果我必須爲這種態度取個簡短的名稱，我會叫它 radical empiricism，儘管這樣簡短的綽號在哲學上常常更加具有誤導性。我稱其爲“經驗主義”，因爲這種態度認爲其對於事實的非常肯定的結論只是一種假設，在未來經驗的過程中可能會被修改； 我稱其爲“激進主義”，因爲它將唯一實體論學說本身視爲一種假設，並且，與其說它像現在流行的博斯莊主義、不可知論或者科學自然主義一樣，半路上的經驗主義在意識形態上認爲唯一實體論是所有體驗都需要證實的。唯一實體論和複數實體論之間的差異或許是哲學上最重要的差異之一。[威廉詹姆斯，來自1897年《相信的意志與其他流行哲學論文集》中的《理性情感》序言]
