Beauty is composed of an eternal, invariable element whose quantity is extremely difficult to determine, and a relative element which might be, either by turns or all at once, period, fashion, moral, passion.

The sentence distinguishes two components of beauty: a timeless, “eternal” core that resists measurement, and a shifting, contingent element shaped by historical moment and human feeling (period, fashion, morals, passions). Read this way, beauty is neither purely objective nor purely subjective: it has a stable aspiration toward the universal, yet it is always encountered through the filters of culture and desire. The formulation is often invoked in modern aesthetics (including film criticism) to explain how artworks can feel both of their time and capable of outlasting it—because they fuse transient style with something harder to define that continues to compel attention.