 Again, looking at the diagram on the side of the singulars, most 13th century authors agreed that what accounts for the specific unity of several individuals of the same species, namely, their specific nature, should be something other than what accounts for their numerical distinctness, namely, their principle of individuation. However, one singular entity in a species of several co-specific individuals has to contain both the principle of the specific unity of these individuals and its own principle of individuation. Therefore, this singular entity, being a composite at least of its specific nature and its principle of individuation, has to be distinct from its specific nature. At any rate, this is the situation with material substances, whose principle of individuation was held to be their matter. However, based on this reasoning, immaterial substances, such as angels, could not be regarded as numerically distinct on account of their matter, but only on account of their form. But since form is the principle of specific unity, difference in form causes specific diversity. Therefore, on this basis, any two angels had to be regarded as different in species. This conclusion was explicitly drawn by Aquinas and others, but it was rejected by Augustinian theologians, and it was condemned in Paris in  1277.[43]