I am sure you have heard of someone described as lacking commonsense..or some other person having commonsense.

"Commonsense ideas tend to relate to events within human experience, and thus commensurate with human scale. Thus there is no commonsense intuition of, for example, the behavior of the universe at subatomic distances or speeds approaching that of light." - Wikipeida.org

Is an axiom the same thing as "commonsense"; what is commonsense to you?

Common sense does not exist, or if it does exist, it is not desirable. What is wanted is not "common" sense, but good sense. To the extent that sense is common, it is seldom very good; to the extent that it is good, it is seldom very common.

OTOH, there is a philosophy of realism to which I subscribe called the commonsense theory (not theory in the scientifici sense) of realism. But that isn't what most people mean when they talk about "common sense."

Usually the people who prate about this don't have good sense and use it as a bludgeon to thrash people who know more than they do! "Oh, you may be smart, but you haven't any common sense, but I have LOADS of common sense."

One of my dictionaries defines Common as follows:

Adjective: com?mon?er, com?mon?est.
1. a. Belonging equally to or shared equally by two or more; joint: common interests.
1. b. Of or pertaining to the community as a whole; public: the common good.
2. Widespread; prevalent; general: common knowledge.
3. a. Of frequent or habitual occurrence; usual: a common phenomenon.
3. b. Most widely known or occurring most frequently; ordinary: the common crow.
4. Without special designation, status, or rank: a common sailor.
5. a. Not distinguished by superior or other characteristics; average: the common spectator.
5. b. Of no special quality; standard: common procedure.
5. c. Of mediocre or inferior quality; second-rate: common cloth.
6. Unrefined or coarse in manner; vulgar: behavior that branded him as common.
7. Grammar.
7. a. Either masculine or feminine in gender.
7. b. Representing one or all the members of a class; not designating a unique entity.

To make it specific to ?common sense? I think is meaningful. Statements like ?it takes three _____ to screw in a light bulb? are examples of pointing out that a particular person or class of people lack the common knowledge of doing that activity- they lack the common sense (of the rest of us). Things which all people are presumed to know are part of the common sense of those people. This can be very local when used in that fashion, such as where the library is located. In my mind I have a more limited interpretation of the term. If I were to suggest some person lacked common sense I would be predicating that statement on some showing by that person having don, not stupid things, but rather unnecessary or wasted efforts demonstrating a lack of common everyday knowledge of things.
jjw

Common sense is what someone who got themselves or others into trouble doesn't have. That is, provided that you would not have done the same thing if you were in that person's place.

Common sense is when people agree with what I deem logical and obvious.

A lack of common sense denotes someone that thinks I am wrong-headed and need to be restrained.

In short: It is a self-serving phrase.

Common sense is the thing that stops people who have it from posting such irrelevant topics.

Who likes my DA Morgan impression?

I do.

In Spain they say "common sense is the least common of the senses".

I would say that common sense includes a notion of realism. Often, children show commoner sense than adults because they are less conditioned by "cognitive filters" (prejudices and projections) when facing the world outside.

I just started a biography of Kurt Godel, and one sentence in the introduction struck me as particularly relevant:

"What, in the long run, is common sense, other than common?"

It is glib but rather meaningless if thrown around without understanding its context. Godel believed that common sense has little to do with reasoning, and where the two areas of thought do converge it is purely coincidental, and not to the credit of common sense. Common sense, as the term implies, is exactly the kind of thought that goes unexamined and therefore has a tenuous connection to objective reality -- an ideal Godel strove for in all his scholarship.

The book is Incompleteness: The Proof and Paradox of Kurt Godel.

I think common sense may exist. It's lacking in aphasia victims, indicating that there is some neurological phenomenon associated with mistaking one's wife for a hat.

VB