Doubt is typically an indication of uncertainty, whereas knowledge typically implies certainty. While there may be exceptions to either, if you're entertaining any doubt (uncertainty) about something, it follows that you do not know it (certainty), since you cannot both be certain and uncertain about the same claim at the same time in the same sense. Thus, doubt is a sword that can be used to cut yourself off from what is not known. For this reason, those who have a vested interest in your believing something will oppose your doubt, making it costly. That's a sign that they want you to outsource your philosophical judgment to them, and that is another reason to doubt. Socrates trusted an inner source that generated hesitation towards certain decisions and actions; he thought its source was divine. Trust your inner philosopher, especially when it raises a doubt. It may be a whisper from a divine source.