 In Ζ.2 he recounts the various answers that have been given to the question of which things are substances—bodies (including plants, animals, the parts of plants and animals, the elements, the heavenly bodies), things more basic than bodies (surfaces, lines, and points), imperceptible things (such as Platonic Forms and mathematical objects)—and seems to regard them all as viable candidates at this point. He does not seem to doubt that the clearest examples of substances are perceptible ones, but leaves open the question whether there are others as well.