Natura - nature actualia - actual entities Mundus Plinius - physical world naturalia - natural bodies "crystal-kingdom" chemical substances chemical compounds Class carboxylic acids - RC(=O)OH propionic acid derivatives - CR3CR2C(=O)OH ibuprofen - (~C=CHCH=C(CH2CH(CH3)2)CH=CH~)CH(CH3)C(=O)OH