Dhīra, one who is cool-headed. Not a passionate, crazy fellow, but cool-headed. Dhīras tatra na muhyati. He can understand that as one passes through different bodies—baby's to childhood, childhood to boyhood, boyhood to youthhood, then old age—similarly, this body, when it will be no more existing, na hanyate hanyamāne śarīre (BG 2.20), the body may be destroyed, but the soul will continue to exist. This is the Vedic principle of knowledge.
This is called spiritual knowledge. Spiritual knowledge does not mean anything else. To understand the spiritual, constitutional position of the living entity, that is called spiritual. And at the present moment, by constitution, my position is that I never die or I never take birth. But because I have accepted this material body, therefore I have to change. Vāsāṁsi jīrṇāni yathā vihāya (BG 2.22). Just like we change our garments. I am putting on this garment. When it is old or not usable, I give it up; I accept another coat or shirt. Similarly, we have got coat and shirt over our position as soul. The shirt is the subtle body: mind, intelligence and ego. And the gross coat is made of five elements: earth, water, fire, air, sky. In these two coverings, I, the soul, I am existing.