Basis of commonsense knowledge assumptions

What is commonsense? A gave a ball to B. Who has the ball now? B. If you are going to someone’s house, you expect there to be air in the hall (and rooms). You don't expect nitrous oxide or carbon monoxide. This is a commonsense assumption you make. This is very very plausible. But what is the basis of this? Statistics (all houses have air in them)? Or Experience (all houses you have been to have had air in them)? Or sheer “Worldliness” (things being the same in this world, across places)? Variables are samples of constants. Constants are common across entities, variables change. A cricketer is a constant, Sachin Tendulkar is a variable. How do you know which is a constant across scenarios? There are two levels of perceiving data - looking at the constant (general) and looking at the variable (specific). And these are levels - the same thing can be a general or a specific according to which level you are looking at the data at. A bag with some name (say, rotary club) written on it is general (bag) as well as a specific (a carrier example). Rotary club bag is a specific (of the bag) as well as a generality of different designs of rotary club bags. The word you use to represent something is critical. That determines its level. Empirically speaking, the natural default form of perceiving and referring to data is ‘general’. So when I enter a gym for the first time I perceive the body builder guiding people, for the first time, as a trainer (general) and then as say Alex or Jim (specific). Across gyms the trainer (general) is a constant, whereas the Alexes and the Jims vary. So when I enter any new gym I expect my constants to be there as they are and the variables to be different. That is, when I enter any gym I expect there to be trainers but not necessarily Alex and Jim. When I enter a house I expect there to be air around (and not nitrous oxide or carbon monoxide) but not the same shape of the dining table as is in say my house. This is the basis of commonsense knowledge assumptions - why and how we make them or consider them as plausible.