Is 'honest' a suitable synonym for 'sincere'?
These are NOAD's versions of the terms:
sincere |sinˈsi(ə)r| adjective ( -cerer , -cerest ) free from pretense or deceit; proceeding from genuine feelings : they offer their sincere thanks to Paul. • (of a person) saying what they genuinely feel or believe; not dishonest or hypocritical.
honest |ˈänist| adjective free of deceit and untruthfulness; sincere : I haven't been totally honest with you. • morally correct or virtuous : I did the only right and honest thing. • [ attrib. ] fairly earned, esp. through hard work : struggling to make an honest living. • (of an action) blameless or well intentioned even if unsuccessful or misguided : he'd made an honest mistake. • [ attrib. ] simple, unpretentious, and unsophisticated : good honest food with no gimmicks.
As you see, the meanings of the two words intersect, but there are real points of difference in some of the senses. There are cases where you would not substitute one for the other. Sincere is hardly going to be understood to mean "simple, unpretentious, and unsophisticated" for example. And honest has more to do with veracity than feelings.
There are a number of shades of difference in the meaning. I'll just mention one that nobody has touched on so far: If you describe somebody as an honest man, this implies he is generally honest. If you describe somebody as being sincere, you are generally talking about a single instance of honesty.
He told me that he would give me the money if he found it. I thought he was sincere.
You shouldn't say honest here, because it's only a single instance of honesty. If you want to use the word honest, you should say:
He told me that he would give me the money if he found it. I thought he was being honest.
Depends on the context.
I didn't eat the chocolate! Honest!
"Sincere" would not be a synonym in this construct.
Tell me your honest opinion about my novel manuscript.
"Sincere" may work here, but "honest" sounds better.
His honest contributions have brought the party to a leading position.
"Sincere" will work here, but it still sounds different from "honest".