“Our business here is not to know all things, but those which concern our conduct.”
Locke places useful limits around intellectual ambition: human inquiry need not comprehend everything in order to guide life well, and knowing where understanding stops is part of using it responsibly.
Source details
An Essay Concerning Humane Understanding, Book I, Chapter I, section 6, Project Gutenberg #10615. Locke follows his sounding-line analogy by saying that our business is not to know all things but those concerning conduct. The passage belongs to seventeenth-century epistemology and moral reflection, not a modern cognitive-science finding.