[Sage]

I realize why Apple keeps the spell checker and built-in dictionary separate (you can teach the spell checker new words, but not the dictionary), but why is there such a huge disparity between them? For example, if you type in “consequentialist”, the spell checker will think it’s a nonsense word, but then press Command-Control-D and you get a perfectly legitimate definition. It seems that if a word is included in the dictionary, it should’ve been added to the spell check list.

[TETENAL]

Why are they different? Have a look at /Library/Documentation/Acknowledgments.rtf.

The spell checker is provided by a company called "Vantage Research" while the dictionary content is provided by "Oxford University Press, Inc." Why don't they synchronise their content? Who knows. Maybe they are competitors and hate each other. Maybe they haven't heard of the other's product. Maybe they don't give a crap.

Now that you know that consequentialist is actually a word, just add it to your custom word list.

[Chuckit]

I agree. The OS should automatically add any word in the dictionary to the spell checker.

[Catfish_Man]

I wonder if it would be possible to write a script to do so...

[Thinine]

Sure, it's just a huge XML file, essentially.