“Not” is an adverb. It negates the verb, adjective, or adverb that it refers to. So far so good. “Not” does not modify nouns or noun clauses.
Cover the bottom sentence in the picture, then read the top sentence, then think about it.
That can’t be right! Discernment has something to do with knowing the difference, right? Now read the bottom sentence. Aha!
Let’s go back to the top sentence. “Not” doesn’t modify “knowing,” it modifies “is”! Having “not” modify the wrong word is an easy mistake to make without the context of having that bottom sentence. To make the top sentence unambiguous, you need to separate the “not” from the noun clause, which happens to start with a present participle. (Present participles are verb forms, so “not knowing” feels okay.)
You could do the separation four ways.
- Put a colon after the “not”:
Discernment is not: knowing the difference between etc.
- Put the noun clause in quotes. Do this to both sentences to make them parallel:
Discernment is not “knowing the difference between etc.” It is “knowing the difference etc.”
- Reorder the sentence:
Knowing the difference between right and wrong is not discernment.
- Speak the sentences, and pause after the “not” in the first sentence.
Myself, I’d repeat the word “discernment” rather than use “it” as the subject of the bottom sentence. But who am I to tell Charles Spurgeon how to write? Of course, Spurgeon was a preacher, and he probably first said these sentences in a sermon, and someone wrote them down to make the poster. So maybe I’m not criticizing Spurgeon after all.