- Never attribute to malice that which is adequately explained by stupidity.
This sentence uses difficult grammar. A student could look at this sentence and think there is a noun phrase which looks like this:
- malice that which is adequately explained
This would be a very odd noun phrase. It would look a bit like a noun followed by a relative clause. But this relative clause would have two relative words that and which. However, as the Original Poster says, this would not be grammatical. We cannot use that and which together as relative words for a single relative clause.
Let's look at how the sentence works. This sentence has an unusual word order. We normally say:
Here X can be a person or a thing:
- We attributed [this quote] [to Shakespeare]
- We attributed [the poor attendance] [to the weather]
If we attributed something to X, it means that we thought this thing belonged to X, or was caused by X. So in the examples above, we thought the quote was Shakespeare's and we thought the poor attendance was caused by the weather.
The Original Poster's example
The example sentence puts these phrases the other way round for special effect (and there are special grammatical reasons why the writer can do this). It uses the structure:
- attribute [to X] [something]
If we put the phrases in the normal order the sentence looks like this:
- Never attribute [that which is adequately explained by stupidity] [to malice].
There the Direct Object of the clause is:
- that which is adequately explained by stupidity
Here the word that is being used like a pronoun. It is the same word we find in sentences like That is beautiful. It is not the same word as relative that. The writer could have used a different pronoun here. He could have used something and written this instead:
- something which is adequately explained by stupidity
We can see from this that the relative clause here is:
- which is adequately explained by stupidity
This relative clause explains what the word that refers to.
The sentence means something like:
Don't blame a problem which can be explained by stupidity, on malice.
In other words, the sentence is saying that if we have a problem we often think that somebody caused that problem to be nasty or to cause harm. However, if it possible that somebody was stupid and just made a mistake, that is a better explanation for the problem.
In everyday speech we don't hear many sentences using that followed by a relative clause. However, we see this happening a lot in literature. Here is a quote attributed to Nietche:
- That which doesn't kill us makes us stronger.
(Personally, I don't think this quote is true. Hiccups don't kill you, but they don't make you stronger! Neither do rabbits)