Full Idea
The simple ideas we receive from sensation and reflection are the boundaries of our thoughts; beyond which the mind, whatever efforts it would make, is not able to advance one jot.
Gist of Idea
The absolute boundaries of our thought are the ideas we get from senses and the mind
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.23.29), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 09.3
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.175
A Reaction
My view is that this is wrong, simply because it takes no account of inference to the best explanation. We reach the boundaries of experience, and then we think about it, and penetrate beyond. His 'reflection' doesn't seem to mean that.