(I have indulged  the common English  translation here for שכיח but obviously much meaning is often lost in translation and it would be more correct to at least use the hebrew equivalent that Chazal use, namely “מצוי” ) 

Our Gemara has been spending a lot of effort in determining the minimum quantity of various things that one would need to transport on shabbos in order to be subject to the relevant penalties.

The general rule that has been established so far is that we identify the minimum amount that is useful for its required purpose and that becomes the cut-off point. 

What happens, however, if something has a common use that requires a certain quantity, but also a less common, or uncommon use that requires a lesser quantity ?

Do we go by the higher quantity that is required for its common use , or are we stringent and go by the lower quantity required for the less  common or uncommon one?

As an example, water is used for many purposes – it is used for drinking , washing, cleaning or soothing wounds, cooking , and much more .

Drinking water is clearly a more common use than using it for a wound , given that people need to drink multiple times a day, but hopefully are not wounded that often  .

Abaya formulates a rule whereby whenever an item has a common and an uncommon use , the common use is the one that counts .

Therefore , even though both wine and milk can be used for healing, for which smaller quantities are sufficient , we go by the quality required for having a meaningful drink, namely a reviis ( or a quarter reviis for wine that needs to be diluted as discussed earlier.)

However, when an item has 2 common uses, we go by the smaller of the 2 quantities.

 ( he does not state whether they need to be equally common however , and whether common is an objective or relative term is an important discussion I hope to be able to address another time, and at least touch on here.)

As a result , when it comes to honey which is commonly used both for healing and eating, we go by the smaller amount  that is useful for putting on a wound .

However, Abayas rule is challenged by our Mishna, which says that the minimum quantity of water is the amount one used for eye ointment, significantly less than the amount used for a significant drink.

It is clear to all that drinking is a much more common usage of water than spreading eye ointment !

Abaya answers by limiting the scope of our Mishna to people in the Galil, who were poor, and would never use wine or milk for wounds.

For them, using water for a wound is thus also considered common, and the lower amount needed for that is what counts as far as shabbos is concerned .

Rava also seems to accept Abaya’s rule, but gives an alternative answer, whereby the scope of the Mishna is not limited but applies to everyone.

His reasoning is that seeing as according to Shmuel, wine and milk can cause long term harm to the eye when used in ointment and water does not, people prefer water for such purposes, and thus healing is also considered a common use of water .

How though do we define “שכיח” or “common” regarding this and other halachot?

Firstly, it is important to clarify that whereas we are indulging the use of the English word common as a translation for the Aramaic “שכיח” , what is important for the sugya is not the Oxford definition of the English word, but the halachik definition of the original Aramaic  word.

It is well known that one of the main criteria Chazal use to decide whether to make a decree against a certain action A in case it leads to a biblically prohibited action B, is how   common this forbidden result in fact is.

It should  also be known to any student of Bava Kama that the amount of liability  one has for damages done by one’s animal depends on how common (מצוי) , the damage is, and thus how much one was expected to guard it .

When it comes to entering dangerous situations , we also know from various sugyos ( for example Pesachim 8)  that when damage is common, a person may not rely on being protected by the Mitzva that he is performing. 

In addition, the requirement to check whether something is not kosher or not ( like checking a slaughtered animal  in various vital organs to see it does not bare signs of being critically injured before slaughter  ( a טריפה), is dependent on how common such injuries are ( see Rashi, Chullin 12a) , as is the requirement to check for bugs in fruit and vegetables ( see Rashba  on same sugya though he admits Rashi disagrees ) – the generally accepted rule there is that a “common” minority or “מעוט המצוי” of problematic  cases is enough to require checking, at a rabbinical level, even though biblically, a majority is required. 

It is not certain that the same definition of “common” would apply to all the above situations, but it certainly would be nice to find more rather than less consistency in the usage of the term – unfortunately I am unable to provide this right now .

However, let us at least try to define the term as much as possible within the scope of our current daf and sugya .

There are two major possibilities that stand out:

1. Common is defined by the person, or people who use the item.  If people  commonly  or perhaps most commonly use an item for purpose A, then even if they also use it for purpose B, we go by A and not B.

2. Common is defined by the purpose  the item serves .

If the purpose is  commonly or possibly  most commonly served by item A, then even if it is also sometimes served by item B, item A’s minimum required quantity is tied to that purpose , even if it has another equally, or possible more common purpose with a higher required amount .

A Nafka Minah would perhaps  be in our case, the case of water , (as well in the case of honey mentioned in this sugya, but I leave that to you to consider .)

If the first definition is used, then the important factor will be what water  is commonly used for. 

However , if the second definition  is used , then the  relevant factor will be whether water is a commonly used item for the purpose that the least amount of it is required for , namely eye ointment .

It seems more likely to me  from the answers of Abaya and Rava, and this is indeed how Rashi seems to learn, that  the second definition is what counts .

Even the people of the Galil, according to Abaya, and all people according to Rava , clearly use water more often for drinking than for wounds .

Yet the fact that when ointment is required for a wound, water is the item of choice and a commonly ( perhaps most commonly ) used item, ointment is considered a common use of water regarding our rule, and the smaller quantity required for it is what counts !

( p.s. The door is not necessarily closed on the first option, assuming that “common” has some objective rather than relative definition, which does not fit water being used for ointment in the הוה אמינא   but does in the מסקנה . Any suggestions are very welcome )