It's a matter of principle.

Meaning

It's based on a strong belief about what is right or wrong.

Examples

• John: 'I refuse to participate in that activity.'
• Alice: '<b>It's a matter of principle</b> for me, too.'
• John: 'I understand; <b>it's a matter of principle</b> for you.'
• Alice: 'I won't compromise on this issue.'