If there were no deities we would have to invent them, but as for the perception of beauty we only have to rely on our senses, which are not as primitive as it might seem, but rather very sophisticated. Knowledge about art varies from individual to individual, it depends on the degree of cultural evolution, and it is terribly influenced by art critics opinion who make the public what they want. But when we are arranging a beautiful flower bed or arranging flowers on the table, everyone is able to recognize when the composition is perfect. When we say to someone: you are beautiful like a God, we know very well what it means and the diversity of opinion affects very little. Regularity in perception, balance, harmony of the single parts, added to that ineffable feeling that nobody can describe... well then we know that we are in presence of beauty.