Metaphysical Contemplation and Speculation

There are many parables and stories from the Buddha's life suggesting he valued above all what is practical and conducive to virtue. He discouraged metaphysical speculation and musings. **Given without knowledge one can only speculate, does Buddhism suggest one should *delay* certain questions until one can empirically get a definite answer?** For example, the jhanas are not immediately accessible, and thinking about them might lead to speculation. **What is speculation in Buddhism? Should one always restrict their attention and cognition to what is immediately accessible?** I guess I could also add: ***How* does one know when one is speculating?**