"Puedo prometer ser sincero, pero no ser imparcial."

Simplified Meaning:

When someone says they can promise to be sincere but not impartial, they mean they can be honest with you and share their true feelings, but they can't guarantee that they won't have personal biases or preferences. Imagine you ask a friend for advice on a big decision, like choosing between two job offers. Your friend can give you their honest opinion about which job they think is better for you, but their advice might be influenced if they have a bias towards one of the companies, perhaps because they had a good experience there or know someone who works there. This quote reminds us that while honesty is important, people often see things through their own personal lens, shaped by their experiences and feelings. As a result, their advice or opinions may be affected by those biases. In your own life, when seeking advice or making decisions, it's good to listen to honest opinions but also to consider that everyone has their own unique perspective that might not be completely neutral.