Indeed...

Indeed, having ideas will not be enough to secure knowledge if-as in the case of human actions-they are obscure, confused, or imperfect. Given faulty memories, we may also fail to achieve knowledge because we are incapable of tracing long chains of reasoning through which two ideas might be demonstrably linked. In a more practical vein, rational knowledge cannot be established upon false principles-such as those borrowed from conventional wisdom.

Finally-in the effort to achieve philosophical or scientific certainty-our efforts to employ reason are commonly undermined by the misuse or abuse of language. [Essay IV xvii 9-13] Most particularly, on Locke's view, it is difficult to secure the reality of human knowledge in any evidence of its conformity with the nature of things themselves.

We readily assume that passively-received simple ideas must be providentially connected with their objects, and since complex abstract ideas are of our own manufacture, it is our own responsibility to ensure their reality by a consistent use of the names by which we signify their archetypes. But complex ideas of natural substances are intended to represent the way existing things are independently of our perception of them, and of this the content of our ideas never provides adequate evidence.

[Essay IV iv] These difficulties trouble all four types of knowledge. Since knowledge of identity and diversity involves only a recognition that particular ideas are distinct, it is immediately evident whenever we have clear and distinct ideas; such knowledge must be among the earliest that any of us ever achieve.

[Essay IV vii 9-11] Even otherwise ignorant beings might well be capable of perceiving the disagreement of ideas at this level, Locke pointed out, so knowledge of "identical Propositions" of this sort is generally uninformative (or "trifling") rather than any genuinely instructive contribution to morality or science.

[Essay IV viii 2-3] The general principle of identity merits no special status among our cognitive states, Locke held, since its particular instances are all either equally obvious or (because of the obscurity of our ideas) irredemable without it.