Full Idea
The notice we have by our senses of the existing of things without us, thought it be no altogether so certain as our intuitive knowledge or the deductions of our reason, ..yet it is an assurance that deserves the name of knowledge.
Gist of Idea
Knowledge by senses is less certain than that by intuition or reason, but it is still knowledge
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.11.03)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.631
A Reaction
This is a clear instance of an acceptance of fallibilism. So one would hope, I think, from an orthodox empiricist philosopher, especially a representative realist like Locke, who as lots of areas where doubts can creep in.