Natural science studies the physical world.
His argument remained focused on material concerns.
物質的 is a more common, everyday word for 'material' or 'physical', whereas 形而下 is a technical philosophical term.
physical sciences; natural sciences
From Chinese 形而下 (xíng'érxià), literally 'that which is below form', used in classical Chinese philosophy to denote the concrete, as opposed to 形而上 (xíng'érshàng) 'above form', the abstract.