If you have rules, you can make decisions only for problems that fit the rules. With principles, you can make any decisions. It's why a system like Agile is so flexible; there are no rules, just the principles outlined in the Agile manifesto.

I've had various sets of design principles through the years, changing them as technology and I have evolved. Here's my latest version, which I defined for the InsureMyTrip Design System:

Principles

These are the principles from which we make design decisions.

Be ethical

• Be honest. Design honest and clear solutions.

Get close

• Listen to and watch users regularly.
• Ask Why often.
• Involve code early. Talk with developers.

Think big

• Design is how it works.
• Focus on objectives. Set target metrics.
• Design systems not pages.

Verify

• Define and test the problem. Design and test the solution.
• Design from research data but not exclusively.
• Design with real content and data.
• Measure the prototype, the beta, and the release.

Keep it simple

• Include only what's essential.
• Don't make them think.

Be polite

• Products should be considerate.
• Make it accessible.
• Finish the last 5%.