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This is a question taken from a aptitude test but I am confused with the answer .

Only fish oil contains Omega 3.
Only foods that contain Omega 3 help with brain development.

Which conclusion can be derived from the combination of these two statements?

All fish oils help with brain development.
Only what contains Omega 3 is fish oil.
All that helps with brain development is fish oil.
There are fish oils that help with brain development.

What would it mean if the word "only" was replaced with "every"

Every fish oil contains Omega 3.
Every foods that contain Omega 3 help with brain development.

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"Only" here is somewhat implicative of "every" in this particular usage (although some might disagree with my interpretation of that) but in general the mean two different things.

Consider a more simple case, where you say something like one of these.

a) Every man is tall.
b) Only men are tall.

As you may or may not notice, there's a pretty significant difference here. In the first, we're saying that if a person is a man, he must be tall. We're disregarding the truthfulness because neither of these statements is actually true, but that's what that statement would mean. On the other hand, the other statement is something of the inverse: if a person is tall, he must be a man. From a purely logical standpoint, neither implies the other. You would have to say something to the effect of "every man is tall and only men are tall," if you wanted to suggest that both are true.

The word "only" is saying that only elements in the described set may match the attribute given, whereas the word "every" is simply indicating that each element in that set does display that attribute.

Fundamentally, "every" is limiting the elements in that set (in this example, "men"), and "only" is limiting the elements outside that set.

Moving back to what you actually asked about, then, the same difference applies.

Only what contains Omega 3 is fish oil.

If you were to replace "only" with "every" (and there are some grammatical errors with doing that as you have, but those are minor and I'll address them later on), you would be changing the meaning. Right now, it's saying that nothing is fish oil, unless it contains Omega 3. If you changed it to "Everything that contains Omega 3 is fish oil," you would be altering the meaning to suggest that something cannot contain Omega 3 unless it is fish oil.

I'm not sure if I've explained this particularly well, but I hope my example helped. Let me know if I can clear anything up. It's actually a fairly complex matter in logical reasoning, and there are college classes that go over these sorts of differences.


I promised a bit on grammar, so here it is. I won't get into detail, just what it should be.

You said:

Every fish oil contains Omega 3. Every foods that contain Omega 3 help with brain development.

But technically, this would have been the correct transformation:

All fish oil contains Omega 3. Every food that contains Omega 3 helps with brain development.

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This is a question in logic, not English usage. If you know what "only " means, and what "every" means, you could solve it with Venn Diagrams. (see, for example http://www.purplemath.com/modules/venndiag.htm )

This would make it quite clear that substituting "every" for "only" drastically changes the meaning; and it would show the result, from which you could see which of the "conclusions" (if any) would be derivable from that rephrasing.

By the way, the answer to "Which conclusion can be derived...?" is "Only the second, third and fourth" (or if you'd rather, "Every one of them except the first one")

But if you changed the "only" to "every" as proposed, then only the first and fourth conclusions could be derived (or if you'd rather, "Every one of them except the second and third ones.")

(I can do this kind of stuff in my head, but if you can't, Venn Diagrams might help.)

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