The aspect of taking ideas seriously that you are talking about seems orthogonal to forming beliefs. It's about initiative in investigating ideas and considering their general applicability, as opposed to stopping at a few superficial observations or failing to notice their relevance in unusual contexts. You don't need to believe an idea to investigate it in detail, the belief may come eventually or not at all. Considering an idea in many contexts may also blur the line with believing it. (Another aspect is taking action based on a belief.)

The process of investigating ideas in detail might get triggered by believing them for no good reason, but there is no need.