Why is a detective called a detective?

-ive 
a suffix of adjectives (and nouns of adjectival origin) expressing tendency, disposition, function, connection, etc.: active; corrective; destructive; detective; passive; sportive.

http://dictionary.reference.com/browse/-ive

One online etymology website says that “detective” comes from a contraction of “detective police.” Using the definition of ”-ive” to mean function, this would mean something along the lines of “police whose job is to detect”.

Ah, okay @bkcunningham, thanks for that info!
But still, does a detective just tend to detect stuff, or is his job to do it, no matter what?
Thank you as well, @PhiNotPi!
Can you please provide me with a link to that etymology website (as I am very interested in where words originate)?

@rebbel It would be his job to detect stuff, such as clues at a crime scene, and to solve mysteries of some sort or another.

To detect facts.

There are also some nuances in the word detect that seem to make it appropriate for the profession in that it can mean to discern things that may be hidden from view at first. And if you look at the latin roots: dētectus meaning “uncovered”, from dētegere , “to uncover”, which is from de + tegere meaning “to cover.” And that is a lot of what a detective does: uncover facts that may be either buried in a lot of information that isn’t pertinent, so they have to sift through it to find clue or uncover facts, information that has been purposely hidden from view by a perpetrator.

As @bkcunningham explained, -ive suffive can denote purpose or function.

In this case the noun (agent, person, policeman, &c.) is so often implied that the adjective has become treated as a noun itself.

Compare to sedative. Saying “it’s a sedative” implies that it is a sedative medicine, or a plant, or perhaps a piece of music, depending on the context.

It is an adjective, but often used like a noun, where the actual noun is implied.

@rebbel Here is the website that I used.

Beats the alternative

I was going to post the same link to the etymology of the word “detective” but I saw that @PhiNotPi had already posted it so I posted a comment about a different aspect of the word but it seems that there might be a little confusion here, and maybe the confusion is mine, and if is I apologize to @rebbel and everyone who has commented so far. I hope this clarifies things for @rebbel rather than muddy the waters. Everyone is pretty much right, in way, and I hope I am right too.

I think @rebbel is right. The word used really should be “detector” rather than “detective.” If you were just making the word up yourself, that would be correct, “detector.” The suffix ”-or” means “one who,” so you would correctly create the word “from scratch” by taking the verb “detect” adding the suffix ”-or” to create a word meaning “one who detects.”

Nominalization is the process in languages whereby words from one word group, one part of speech, nouns, verbs, adjectives, e.g. can transformed into or become words in another word group: nouns can be turned into verbs, adjectives into nouns, etc.

There are a number of ways an adjective can become or be transformed into a noun. You can add a suffix as the nominalization entry above describes. “careless” becomes “carelessness,” “happy” becomes “happiness” or “intense” becomes “intensity” and “difficult” becomes “difficulty.”

There is another process called “conversion or zero-derivation”:
http://en.wikipedia.org/wiki/Conversion_(linguistics) where a word is simply permitted to be used as another word form without any changes at all. I’m not sure how that one comes about.

Another process is described in this article about adjectives, and if you look at the etymology of “detective” this seems to be what happened, how the adjective “detective” became a noun.

Adjectives are used to qualify/determine nouns, therefore their determined nouns must be present explicitly (or implicitly, via some pronouns). However, there are instances when qualifying/determining adjectives are missing their determined nouns; consequently, these adjectives become:

A. nouns
B. pronouns

iIf you look at the link to etymology of the word that @PhiNotPi posted it says:

1850, short for detective police, from detective (adj.), 1843; see detect + -ive.

So, the word was once an only an adjective describing a certain type of police. Then it sort of “lost” it’s determined noun, “police,” as mentioned in the description above and that’s how and when it became a noun, in 1850 to be precise.

This is also, it appears how the word “sedative,” the example @The_Idler gives, became a noun. It used to be just an adjective desribting a class of druges, “sedative drugs,” then it lost its determined noun and became a noun itself.

sedative (adj.) “tending to calm or soothe,” early 15c., from M.L. sedativus “calming, allaying,” from pp. stem of sedare (see sedate). The noun derivative meaning “a sedative drug” is attested from 1785.

I hope this doesn’t muddy the waters but maybe clarifies things. Also, I haven’t really proofread this post so there may be errors. Sorry, I hate when I make grammatical errors in a post about grammar.

Thank you all (thank you specifically, @lillycoyote, your second answer wasn’t making it muddy atall; instead, for me it was very clear and informative!) for your answers!
I learned something from it, I appreciate that!