I think there is some misunderstanding in what informal logic is. First of all, it doesn't deal with facts, whether the facts are derived intuitively or imperically. The examples you provided...
quote:
2 + 2 = 4 makes sense, and could be "common".
the earth is flat makes sense, and is "common".
have nothing to do with informal logic. These are either known or unknown facts.
Informal logic deals directly with how language works and how a conversation can make sense to everyone. In other words, informal logic deals more with how facts backed up rather than what the facts are.
For example, 2+2=4 does not involve logic, but statement like "2+2=4 because it is a mathematical axiom." Another logical statement is "The Earth is flat because all our observations seem to indicate that it is flat and there is no evidence that we know of that say otherwise." Simply saying the Earth is flat is not logic. It is simply an assertion.
So, in response to your suggestion, I would say that logic, whether it is formal or informal, is very different than common sense. Logic deals with conversation and how facts are presented while common sense deals more with one's own intuition of what the facts are.
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