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Is there even a difference between the two?

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4 Answers 4

My thought is that "purpose of" refers to an intended function of an item. "Purpose for" refers to how an item will be used, regardless of that use's relation to the item's intended function.

Hmm. I'm not describing that well. How about this:

The "purpose of" a shoe is protecting your feet. A possible "purpose for" a shoe is to smash bugs.

So "purpose of" describes a property or capacity of a shoe, where "purpose for" describes what might be done with a shoe.

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This is an interesting topic. To add to bikeboy389's response: the purpose of something is the reason it exists. The purpose behind something is the reason in the mind of whoever was responsible for it. My intuition is that a purpose for something is more likely to be a use or reason associated with it after the fact, or subject to debate.

Found via Google:

Providing Students With a Purpose for Reading

i.e., giving students a purpose/reason to read; and

following God's purpose for our lives

i.e., an individual/agent (God) has designated a purpose for something. (The purpose of our lives would frame it as inherent rather than designated, I think. *God's purpose of/behind our lives would be ill-formed, but the purpose of God for our lives is OK in the right context.)

(There are also contexts in which you can use the construction purpose to a thing, e.g. He embraced religion in the hope that it would reveal the purpose to our lives. This feels fixed or archaic compared to the other options.)

You'd say a/the cause of something to refer to its cause. Having (just) cause for an action, though, is a different construction: it means you have sufficient grounds/justification to perform that action, esp. in a legal sense: The judge's flawed instructions are cause for an appeal.

On the other hand, I don't think reason works in the same way: e.g. a/the reason for getting up early and a/the reason to get up early are fine, but *reason of getting up early sounds wrong to me. You could also say the reason for/behind the decision. Words behaving similarly to reason: rationale and motivation.

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For me, it feels right to use of when purpose is the subject of a sentence or a clause:

The purpose of a hack saw is to cut through small pieces of metal.

And, also for me, it feels right to use for when purpose is the object of the sentence or clause

This is an odd-looking tool, but I'm sure we can find a purpose for it.

EDIT: On the other hand, it just occurred to me that it would also be fine to say:

This is an odd-looking tool we found at the dig site. I wonder if we'll ever figure out the purpose of it.

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I'm still thinking of an answer for this one. I agree it's a matter of feel --- one of those things I never really think about. In your last example, though, I'd certainly prefer "its purpose" rather than "purpose of it"! –  Jimi Oke Dec 17 '10 at 20:13
    
@Jimi Oke: Let's take "it" out of the equation: "I wonder if we'll ever figure out the purpose of this tool." Same construction. We could say "this tool's purpose" but we wouldn't have to. What if I said: "I wonder if I'll ever figure out the purpose of my time here on earth." I think it would sound very awkward to say "my time here on earth's purpose." –  Robusto Dec 17 '10 at 20:17
    
Of course, "it" is preferred in the situations you mentioned. I was only specifically referring to the third of your three examples. Consider, for instance, "What is the purpose of it?" This is a question that could possibly arise from an unpopular suggestion or in reaction to some instruction or course of action. I would wholeheartedly go for this as much as, or even rather than, "What its purpose?" It's also interesting how "its purpose" and "purpose of it" can connote slightly different meanings. –  Jimi Oke Dec 18 '10 at 7:16

When you use "purpose of", the reader is expecting an abstract rationale. When you use "purpose for", the reader is expecting a more concrete example. Compare:

"The purpose of this website is to educate."

The abstract idea is "to educate".

"The purpose for this website is to get people talking about English."

Concrete: "get people talking". Saying "purpose of this website is to get people talking" in the last example wouldn't sound quite right, although it would still be grammatically correct.

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