Each of these will serve you when applied correctly. Each of these will destroy you when applied incorrectly.
If you’re debating right and wrong, assume your counterpart is brilliant.
If you’re trying to solve a problem, assume your team is brilliant.
If you’re figuring out how to live together, assume your partner is brilliant.
And,
If you’re building a smartphone app, assume your users are stupid.
If you’re teaching a complex physics problem, assume your students are stupid.
If you’re driving on the belt parkway at rush hour, assume your fellow commuters are stupid.
Each of us is both brilliant and stupid. Success comes when you recognize when.