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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.