It is a common phrasing of a comparison. A is to B as C is to D.
Interested in Language
Can someone explain what "is to" means in the following sentence?
: Depression is to moodiness as a hurricane is to a whirlwind.
I see the above construction is parallel, but what makes me frustrated is that I can't find any explanation or definition of "to" in this usage. It seems somewhat formal and literal.
I am not a teacher.
What is compared in this sort of construction is the relationship between the elements of each pair under consideration.
We can all see that there are similarities between depression and moodiness on the one hand and hurricanes and whirlwinds on the other. What the 'A is to B as C is to D' tells us is that the same type of relationship exists in both pairing; in this case degrees of severity.
This kind of construction is an analogy.
In it's simplest form:
'Boy is to girl as man is to woman.'