Idiom is defined by the Oxford English Dictionary as
"a peculiarity of phraseology approved by usage and often having a meaning other than its logical or grammatical one".
When anything in this book is called "good English idiom" or "idiomatic", what is meant is that usage has established it as correct. Idiom does not conflict with grammar or logic as a matter of course; it is usually grammatically and logically neutral. Idiom requires us to say aim at getting, not aim to get, and try to get, not try at getting. Logic and grammar do not object to this, but they would be equally content with aim to get and try at getting. At the same time idiom is, in Jespersen's phrase, "a tyrannical, capricious, utterly incalculable thing", and if logic and grammar get in its way, so much the worse for logic and grammar. It is idiomatic —at least in speech — to say "I won't be longer than I can help" and "it's me". That the first is logically nonsense and the second a grammatical howler is neither here nor there; idiom makes light of such things.
Do not therefore listen too readily to those who say that something "must" be wrong because it is illogical or ungrammatical. If it has not established itself as idiom, then by all means let logic and grammar have their way, or there will be anarchy in the language. But grammar has no chance, and ought to have no chance, against genuine idiom. Logan Pearsall Smith says:
Plainly a language which was all idiom and unreason would be impossible as an instrument of thought; but all languages permit the existence of a certain number of illogical expressions: and the fact that, in spite of their vulgar origin and illiterate appearance, they have succeeded in elbowing their way into our prose and poetry, and even learned lexicons and grammars, is proof that they perform a necessary function in the domestic economy of speech. —Words and Idioms Constable & Co.