A brain extension. Organise your thoughts
New Note
One person's common sense is different to another's... the term is simply shorthand for your own prejudices; usually that you are not aware of as prejudices. For example, "You should close your borders to stop terrorists coming it - it's common sense" or "It's common sense that relationships should be between men and women" or "It's common sense that if more people had guns to take down a shooter, less people would get shot" etc.

This is why one should always be wary as soon as "Common Sense" is stated as justification... it says "I have no real justification, other than my own feeling on it".

Also good to be aware of this if you ever find yourself using the phrase.