Full Idea
The guiding principle is that what is true of one piece of copper is true of another; such a guiding principle with regard to copper would be much safer than with regard to many other substances - brass, for example.
Clarification
Copper is a an element; brass can come in different proportions in the mixture
Gist of Idea
What is true of one piece of copper is true of another (unlike brass)
Source
Charles Sanders Peirce (The Fixation of Belief [1877], p. 8)
Book Reference
Peirce,Charles Sanders: 'Philosophical Writings of Peirce', ed/tr. Buchler,Justus [Dover 1940], p.8
A Reaction
Peirce is so beautifully simple and sensible. This gives the essential notion of a natural kind, and is a key notion in our whole understanding of physical reality.
Related Idea
Idea 8153 By knowing one piece of clay or gold, you know all of clay or gold [Anon (Upan)]