I’d like to offer you, arguably, an uncommon definition of common sense. A definition that I myself refer to as “a common sense definition of common sense.”
Common sense is (literally) what can be commonly sensed. Not thought or felt but sensed. If you and I are both sensing (seeing) a sunrise, that’s something that we are commonly sensing (by way of seeing). As for the rest of what we typically consider “common sense” – it’s really non-sense, i.e. non-sensible abstractions, not actual things we sense (by way of seeing, touching, hearing, or tasting) but the “thing-less things” we think (i.e. mentally envision, imagine, speculate).
“Sense” is the operative word here.
Redefining common sense in terms of the shared sensory experience (as opposed to defining common sense as the shared fund-of-knowledge or shared belief system or shared living wisdom) allows us to downgrade our assumptions about others’ mind-ware (others’ “operating system”) to a minimum. This kind of un-presumptuous stance can be helpful in minimizing misunderstanding early on in the human encounter.
To sum up, we commonly share the sensory experience (the sensory input) but we don’t always share our interpretations of this sensory input. So, rather judgmentally, we conclude: “So and so doesn’t have a common sense.” I think that’s unkind: the reality is that so-and-so does probably share the same sensory experience (same sensory input more or less) as we do. What’s different is what they think about it, how they interpret the shared sensory data-set. To think differently is not to lack common sense but merely to differ, which is entirely normal and to be expected.
Thus, this view of common sense that I suggest here offers a sensible (realistically-minimal) and non-perfectionistic expectation about others’ common sense.