Flipping the bird at answers

Here is a Bertrand Russell quote that constitutes my favourite aphorism:

The essence of the liberal outlook lies not in what opinions are held, but in how they are held: instead of being held dogmatically, they are held tentatively, and with a consciousness that new evidence may at any moment lead to their abandonment.

I don’t know where it’s from – it was quoted in another book, the name of which I have also forgotten. Do you know where it’s from? And what do you think of it?