In response to the 'chimeras' objection, Locke distinguishes verbal truth from real truth and argues… — John Locke

In response to the 'chimeras' objection, Locke distinguishes verbal truth from real truth and argues that truth is real only when the words in a proposition stand for ideas that agree with the reality of things or are capable of existing in nature; otherwise, propositions are merely verbally true and of limited value.

By John Locke, from An Essay Concerning Human Understanding

Key Arguments

He notes that his earlier distinction between 'real and imaginary knowledge' can be applied 'to distinguish real truth from chimerical, or (if you please) barely nominal, they depending both on the same foundation.'
He concedes that 'our words signify nothing but our ideas', but stresses that they are 'designed by them to signify things', so the truth of propositions will be 'only verbal, when they stand for ideas in the mind that have not an agreement with the reality of things.'
He explicitly introduces 'the distinction of verbal and real' for truth, defining 'that being only verbal truth, wherein terms are joined according to the agreement or disagreement of the ideas they stand for; without regarding whether our ideas are such as really have, or are capable of having, an existence in nature.'
He defines real truth as occurring 'when these signs are joined, as our ideas agree; and when our ideas are such as we know are capable of having an existence in nature.'
For substances in particular, he insists that we 'cannot know' that our ideas are capable of existing 'but by knowing that such have existed', thereby tying real truth about substances to experience of actual existence.

Source Quotes

Answered, “Real truth is about ideas agreeing to things.” Though what has been said in the foregoing chapter to distinguish real from imaginary knowledge might suffice here, in answer to this doubt, to distinguish real truth from chimerical, or (if you please) barely nominal, they depending both on the same foundation; yet it may not be amiss here again to consider, that though our words signify nothing but our ideas, yet being designed by them to signify things, the truth they contain when put into propositions will be only verbal, when they stand for ideas in the mind that have not an agreement with the reality of things. And therefore truth as well as knowledge may well come under the distinction of verbal and real; that being only verbal truth, wherein terms are joined according to the agreement or disagreement of the ideas they stand for; without regarding whether our ideas are such as really have, or are capable of having, an existence in nature.

Though what has been said in the foregoing chapter to distinguish real from imaginary knowledge might suffice here, in answer to this doubt, to distinguish real truth from chimerical, or (if you please) barely nominal, they depending both on the same foundation; yet it may not be amiss here again to consider, that though our words signify nothing but our ideas, yet being designed by them to signify things, the truth they contain when put into propositions will be only verbal, when they stand for ideas in the mind that have not an agreement with the reality of things. And therefore truth as well as knowledge may well come under the distinction of verbal and real; that being only verbal truth, wherein terms are joined according to the agreement or disagreement of the ideas they stand for; without regarding whether our ideas are such as really have, or are capable of having, an existence in nature. But then it is they contain real truth, when these signs are joined, as our ideas agree; and when our ideas are such as we know are capable of having an existence in nature: which in substances we cannot know, but by knowing that such have existed.

And therefore truth as well as knowledge may well come under the distinction of verbal and real; that being only verbal truth, wherein terms are joined according to the agreement or disagreement of the ideas they stand for; without regarding whether our ideas are such as really have, or are capable of having, an existence in nature. But then it is they contain real truth, when these signs are joined, as our ideas agree; and when our ideas are such as we know are capable of having an existence in nature: which in substances we cannot know, but by knowing that such have existed. 9.

Key Concepts

though our words signify nothing but our ideas, yet being designed by them to signify things, the truth they contain when put into propositions will be only verbal, when they stand for ideas in the mind that have not an agreement with the reality of things.
And therefore truth as well as knowledge may well come under the distinction of verbal and real;
that being only verbal truth, wherein terms are joined according to the agreement or disagreement of the ideas they stand for; without regarding whether our ideas are such as really have, or are capable of having, an existence in nature.
But then it is they contain real truth, when these signs are joined, as our ideas agree; and when our ideas are such as we know are capable of having an existence in nature:
which in substances we cannot know, but by knowing that such have existed.

Context

Book IV, Chapter V, §8, where Locke directly answers the worry that his account reduces truth to chimerical propositions by distinguishing merely verbal from real truth based on conformity to reality and possible existence.