Full Idea
That nature will follow tomorrow the same laws that she follows today is a truth which no man can know; but in the interests of cognition as well as of action we must postulate or assume it.
Gist of Idea
We can't know if the laws of nature are stable, but we must postulate it or assume it
Source
William James (The Sentiment of Rationality [1882], p.39)
Book Reference
James,William: 'Selected Writings of William James', ed/tr. Bird,Graham [Everyman 1995], p.39
A Reaction
The stability of nature is something to be assessed, not something taken for granted. If you arrive in a new city and it all seems quiet, you keep your fingers crossed and treat it as stable. But revolution or coup could be just round the corner.