Sound philosophy originates in common experience and opinion. It is futile to seek proofs of such things as that we exist; that time passes; that I remain myself even if my hair changes color; that some things are good; or that good is to be pursued.
But if philosophy draws itself from common opinion, then it is not at first clear how philosophy can go beyond it. Why not just bring in the man on the street and ask him what he thinks about things?
In a way, that is exactly what philosophy does. Look how Aristotle investigates happiness, beginning with the common recognition that every act is undertaken for the sake of some end, and then asking questions about this end. Even his scrutiny of common opinions depends on common opinions, for he doesn't simply shoot arrows at them from on high. He makes them interrogate themselves.
For example, one common opinion is that happiness is honor, the praise of other people. Do you say happiness is sought for its own sake? Yes. Then if happiness is honor, honor would be sought for its own sake, correct? Yes. Then would you be satisfied if other people praised you for qualities you knew you did not have? No; that sort of praise has no savor. But in that case, you don’t consider praise worth having for its own sake after all, do you? More fundamental is praiseworthy qualities.
That is how philosophy goes. It can elicit what we didn’t know we knew – but we must already have known it. It can draw inferences -- but we must already grasp the consequence relation. It can call attention to inconsistencies – but we must already realize that contrary views cannot all be true at once. The idea is not to destroy common opinion, but to cleanse, extend, and ennoble it, by forcing it, through dialogue, to be honest with itself.