Examples are a cloud, a human body, a banana, a billiard ball, a table, or a proton. This is contrasted with abstract objects such as mental objects, which exist in the mental world, and mathematical objects. Other examples that are not physical bodies are emotions, the concept of " justice ", a feeling of hatred, or the number "3". In some philosophies, like the idealism of George Berkeley, a physical body is a mental object, but still has extension in the space of a visual field.