Belonging to the realm of reason, intellect, or spirit—the opposite of merely physical or ritual. Paul calls believers to offer 'reasonable worship' (Romans 12:1), spiritual service that engages the mind and will, not just external ceremony. Peter urges Christians to crave 'spiritual milk' (1 Peter 2:2), nourishment for the inner person. Classical Greek used it for organs of speech, prose versus poetry, intellectual virtues versus moral ones. In biblical contexts, it often carries overtones of what's fitting for rational, spirit-indwelt beings.