I think the answer has to be - don't judge by appearances.
And that means not just people, but what I read, what I see around me, what others tell me. Whatever the outward appearance, there's always a lot more going on than meets the eye. There are hidden agendas, personal secrets, crippling traumas, grand ambitions. All sorts of things that lurk behind what's immediately visible.
I try not to judge by appearances, but it's so easy to do, especially when all around me people are doing just that, as if it's perfectly normal behaviour.
We judge people by their colour, their accent, their clothing, their sex, where they live, what job they do, what paper they read - a dozen things that can give us a completely false impression of who they are.
Someone can look blissfully happy and fulfilled when underneath they find their life totally frustrating and soul-destroying. Someone can look desperately poor in their shabby, worn-out clothes, when in reality they're worth millions.
I'm constantly surprised by something a person happens to reveal, something quite at odds with what I thought I knew about them, and I realise I've completely misjudged them all along.
I'm often misjudged myself, given all sorts of traits I've never had, like smugness, aloofness, condescension and stubbornness. but we like to pin people down, don't we? Oh yes, she's this and she's that, you only have to look at her....
There are plenty of people who look like saints and turn out to be mass murderers. And vice versa.