What can be seen but not seen and heard but not heard and alive but not dies?

The riddle likely refers to "thoughts" or "ideas." They can be conceptualized or understood (seen) but are intangible and cannot be physically observed (not seen). They can be expressed verbally or mentally (heard) but lack a physical sound on their own (not heard). Thoughts exist in the mind, giving them a sense of being alive, yet they don't have a physical form that can die.