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There are two examples of the same sentence below, one using “that” and the other “which.”

-To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are underinvestigation.

-To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.

Which is the correct sentence, and what is the general rule?

The second version of that sentence, using ‘that’ is correct.

Let’s see why.

When To Use “That” and When To Use “Which”

Before clarifying the “that”/”which” rule, just a reminder that “who” should always be used when referring to people.

-The boy who threw the ball.

-This is the woman who always wears a black shawl.

When referring to objects, though, the rule for using “that” and “which” correctly is simple:

THAT should be used to introduce a restrictive clause.

WHICH should be used to introduce a non-restrictive or parenthetical clause.

If that leaves you more confused than when you began this article, read on…

A restrictive clause is one which is essential to the meaning of a sentence – if it’s removed, the meaning of the sentence will change. For example:

-Chairs that don’t have cushions are uncomfortable to sit on.

-Card games that involve betting money should not be played in school.

-To our knowledge, it is the only body in the solar system that currently sustains life…

A non-restrictive clause can be left out without changing the meaning of a sentence. Non-restrictive clauses are either in brackets or have a comma before and after them (or only before them if they come at the end of a sentence):

-Chairs, which are found in many places of work, are often uncomfortable to sit on.

-I sat on an uncomfortable chair, which was in my office.

Why You Need to Use “That” or “Which” Correctly

Changing that to which or vice versa can completely change the meaning of a sentence. Consider the following examples:

-My car that is blue goes very fast.

-My car, which is blue, goes very fast.

The first sentence uses that – suggesting I own more than one car (and even implying my other cars might not be so fast). This is what happens if we leave out the clause and write:

-My car that is blue goes very fast.

-My car goes very fast.

The sentence’s meaning has changed: the reader does not know which one of my cars goes very fast.

However, the sentence using ‘which’ simply informs the reader that my car is blue. We can take the clause out without losing any essential information:

-My car, which is blue, goes very fast.

-My car goes very fast.

“That” and “Which” in Common Usage

It is common today for which to be used with both non-restrictive and restrictive clauses, especially in informal contexts:

-Who ate the cake that I bought this morning?

-Who ate the cake which I bought this morning?

The clause “that I bought this morning” is essential to the meaning – I’m not asking about a cake which I bought yesterday, or this afternoon. Therefore, the first example using “that” is the correct one, but many people would not consider the second ungrammatical.

It is, however, incorrect even in informal contexts to use ‘that’ for a non-restrictive or parenthetical clause. For example, these sentences would be considered incorrect:

-This computer, that I have never liked, is very slow.

-The blue desk, that my father gave me.

An easy way to watch out for these is to look for instances where you have a comma followed by the word ‘that’.

Even though the usage of which has been relaxed to some extent, it is still better to keep your writing as clear as possible by using ‘which’ for only non-restrictive clauses, and ‘that’ for restrictive ones.

Let’s get back to the example at the beginning:

-To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are under investigation.

-To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.

The second sentence, using ‘that’ is correct, but many people would consider the first sentence permissible, too. In a formal context such as a scientific paper, it is better to use ‘that’ for total clarity.

source: dailywritingtips.com