Qi (气) is the fundamental life force or energy that animates everything in Chinese philosophy. It's not mystical smoke — think of it as functional energy. When a tree grows, that's Wood qi. When your heart beats, that's Fire qi.
Western science doesn't use the word "qi," but it studies the same phenomena through different frameworks — bioelectricity, circadian rhythms, seasonal biology. You don't need to "believe" in qi to use BaZi. Just treat the Five Elements as archetypal patterns — symbolic categories that describe real human temperaments and life rhythms. The map works whether you think the roads are made of asphalt or energy.