That which or what? 
Question:
In the following example, is it better to write 
that which or 
what?
- The agency will determine what or that which is reasonable.
 
	Answer:Michael Swan, author of 
Practical English Usage (1995), was the only one among the various authors consulted who had anything to say on this subject. Swan considers 
that which to be an older form of 
what. In fact, he says that the form 
that which is very unusual in modern English. However, a search in Google shows that there are over 1.5 million hits for 
that which; surely, that many sources can't be completely off the mark!
It's really a question of style, so be sure to take into account the tone and the level of language you are aiming for. That which isn't wrong, it's just more formal. If you're striving for a lighter writing style, stick with what.