"In and of itself" is a phrase I find myself using all the time. But in and of itself, the phrase "in and of itself" has no meaning. That is, the individual words don't seem to contribute to the whole meaning of the phrase.
The <thing>, in and of itself, ...
means the same as
The <thing> qua <thing> ...
"In and of itself" is very old, and the usages of in and of from which it springs are very rare now (in itself is merely rare on the ground; of itself seems to be nearly extinct in those meanings), but if it helps you can expand it to "in its essence and of its nature".
Words and phrases often hang on long after the things they originated from have vanished. You may know a few people who are ruthless, for instance, but when was the last time you described anyone as ruthfull?
It's a very stylish turn of phrase when used correctly. It's a way of showing a delicate relationship between things. For instance:
- The necktie was not, in and of itself, the cause of his strangulation.
- The necktie was not the cause of his strangulation.
In the first example, the tie is being acknowledged as being a factor in the event, but not a major or causative agent. In the second example, the tie is being completely ruled out. It has a slight nuance of removing blame from the item in question.
I would challenge the premise of the question, as well as parts of the proferred answers.
Taken separately, the elements of “in and of itself” can be pictured as having their meanings borrowed from examples like the following:
The clock stopped of its own accord. (vs. of itself)
The work was complete in some respects. (vs. complete in itself; that is, did not require or depend on additions).
The combination of the two seems merely to serve as intensifier, as in the similar stock phrase, part and parcel.
As for etymology, my guess is that the phrase originated in a calque from the Latin per se, which means the same thing. For a long time English grammarians loved to emulate Latin, and in itself, of itself, or by itself may have been popularized as a sort of direct translation. Though intrinsically may be simpler and more elegant and easier to pronounce to modern English ears than the stuffy sounding in and of itself, there is surely no simper way of saying per se in Latin than per se. This may explain the longevity of in and of itself up to the time when the well-educated stopped automatically learning Latin as part of their education.
Finally, contrary to @bye,
The thing, in and of itself, . . .
has quite a different meaning than
The thing qua thing . . .
Qua means “in the role of” or “under the aspect of”, as in this quote from dictionary.com:
This might be thought a decisive objection to a federal judge’s writing about this subject even if the judge writes qua academic rather than qua judge. ―Richard A. Posner, An Affair of State
To take an “X qua X” example, one might say,
A Clockwork Orange works much better as a book qua storyline-for-a-movie than as a book qua book.
Or, as someone else in this forum quoted,
Bobby was the first pig I had met qua pig, not qua pork.
Finally, to truly come full circle, per se has seemed to slip from its original meaning of in itself / by itself (that is, separated from its context) and now just flabbily refers to a kind of lack of clarity. “I don’t agree with him per se” is now used to mean “I don’t especially agree with him, but I don’t especially disagree either.” The effect of this is to undermine the usefulness of the original phrase. Thus, the sentence “Guns are not bad per se; it’s what people do with them that is” can now be parsed two ways. It can either mean that guns are not intrinsically bad, or that they are not especially bad. As far as I can tell, there is no way to force the first meaning with falling back to some subtitute like intrinsically, or—you guessed it—in and of themselves.
It comes from phenomenological philosophy (think Heidegger) and existential philosophy (think Sartre), in which being "in itself" and being "of itself" or "for itself" have important distinct meanings. The contrast with "of itself" is more associated with Heidegger; the contrast with "for itself" with Sartre. Here is a good explication of Sartre's usage:
[Sartre's Being And Nothingness] begins by analyzing two distinct and irreducible categories or kinds of being: the in-itself (en-soi) and the for-itself (pour-soi), roughly the nonconscious and consciousness respectively, adding a third, the for-others (pour-autrui), later in the book.
Being-in-itself and being-for-itself have mutually exclusive characteristics and yet we (human reality) are entities that combine both, which is the ontological root of our ambiguity. The in-itself is solid, self-identical, passive and inert. It simply “is.” The for-itself is fluid, nonself-identical, and dynamic. It is the internal negation or “nihilation” of the in-itself, on which it depends. Viewed more concretely, this duality is cast as “facticity” and “transcendence.”
You can also find information here: https://en.wikipedia.org/wiki/Being_in_itself
That is why the phrase seems like such a fussy and erudite phrase: it is originally associated with academics wishing to emphasise something in its existential totality, both in itself (physical) and of / for itself (metaphysical).
There is no reasoning behind and of, which is redundant. It is perfectly sufficient to say in itself. It does not even have the near assonance of each and every. The expression implies that there are usually additional factors to be considered. Incidentally, I wonder whether the full expression is in fact that old in English. It looks like a translation of the German an und für sich.