Beyond opinion there is nothing. The objections to this saying of Monimus the Cynic are obvious. But obvious also is the utility of what he said, if one accept his pleasantry as far as truth will warrant it.
Monimus, another slave turned philosopher, a cynic who was famous for “everything is vanity”. All that is inside of us is our opinion, all that is outside is our perception. Dorm room philosophy perhaps, but also true.