This paper analyzes Thomas Aquinas's theory of species by focusing on celestial bodies (e.g. the sun and the moon) and angels. Both share a common characteristic that each individual is specified due to its own specific nature, i.e., each individual is also a species by itself. According to Summa theologiae I.13.9, this individualism is expressed in terms of specific commonness or non-commonness. Whereas the specific nature of angels is incommunicable not only in reality, but also in ratio, that of celestial bodies is not common in reality, but it is common in ratio; it can be multiplied into many individuals under the same specific nature. Our analysis intends to provide us with a detailed explanation of the theory of species in celestial bodies and angels. We will indicate five common points that can be regarded as the principal reasons for their individualism: perfection of the universe, incorruptibility, self-sufficiency of operations, specification of operations and their effects, and specification of motions. On the other hand, the difference between the two kinds of beings involves materiality and immateriality, the former of which gives a logical possibility of quantitative division into many individuals under the same species.
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