"But when, in the first setting out, he takes it for granted without proof, that distinctions found in the structure of all languages, have no foundation in nature; this surely is too fastidious a way of treating the common sense of mankind"

In this quote, theorist Thomas Reid critiques a methodological technique to understanding language that he finds extremely dismissive and speculative. By "setting out", Reid describes the initial presumptions or facilities that a person uses as a foundation for additional examination or argumentation. In specific, Reid is addressing the presumption that the distinctions discovered in the structure of all languages lack any natural basis. Basically, he recommends that beginning with the facility that language differences are approximate or synthetic-- and not grounded in nature or truth-- does not have adequate reason and dismisses the inherent wisdom or common understanding shared by humanity.

Reid's use of the word "fastidious" communicates a sense of excessive daintiness or an interest in minor information that may lead one to ignore wider truths. He indicates that the overly crucial or refined manner of turning down the natural basis of language differences does an injustice to human good sense, the user-friendly and cumulative understanding possessed by people across various cultures and societies. Reid seems to argue for a more well balanced technique; one that recognizes the possibility that the structure of languages could have a foundation in human nature or in the nature of truth itself.

Reid's review likely comes from his broader philosophical point of view, which emphasized sound judgment viewpoint-- a school of idea that sought to highlight the reliability of common human experiences and perceptions as a foundation for knowledge. He likely saw the termination of natural distinctions in language as a move away from this fundamental belief in the credibility of common experiential understanding. For Reid, acknowledging that common languages might show genuine differences in nature gives credit to the shared understanding amongst individuals and highlights that our linguistic structures aren't merely approximate but possibly grounded in shared elements of the human condition.